So, I was about 75% through with collecting data on the teams of every trainer in the game before it occurred to me that someone else has probably already done it. But nonetheless, I've already collected my info anyway and might as well share it. So here's everything you should ever want to know about the enemy trainers you can have rematches with.
Note: These are the trainers' upgraded teams. Some trainers will not level up their teams this far until you've hit the triggers that will allow them to, such as beating the Elite 4, obtaining the National Dex, and catching Heatran. If you haven't done those things, then be sure to check how far the trainers will go on your own copy of the game, by having multiple matches with them to see how far their levels
will go. It's better than before blindly jumping in with Pokemon you really want to train and risking the possibility of needing berries to reverse your mistake.
Short Attention Span Version (AKA where to go and who to fight)
- HP: Route 213. Tuber M Jared gives 6 EVs, and Tuber F Chelsea is right next to him and gives 3.
- Attack: Route 203 for 8 EVs from Youngster Dallas if you don't mind switching out of Golem. Otherwise, go to Cycling Road for 6 Attack from Cyclist John.
- Defense: Route 207 or 228. Hiker Kevin and PKMN Ranger F Krista each give 4 EVs.
- Special Attack: Rainy Route 212's Scientist Shaun nets 5 EVs. If you don't mind needing a switch out of Lickilicky, Route 215's Ace Trainer F Maya grants 6 EVs.
- Special Defense: Beauty Devon, Route 214, 6 EVs.
- Speed: On the west side of Route 211, Ninja Boy Zach for 8.
- Money: Route 212. Gentleman Jeremy or Socialite Reina for 24,400 each.
Long Attention Span Version (where to go, who to fight, where they are on the route, and what advantages or disadvantages they have relative to other trainers giving the same EVs; also lists more options than the above version)
HP
On the beaches of
Route 213,
Tuber M Jared and
Tuber F Chelsea await to give
6 and 3 HP EVs respectively from teams of three Gastrodon and an Azumarill. Nothing else even comes close to having these two standing within range of each other, though since they're in the 50s for their level range, you'll need at least moderate power. Gastrodon takes Double Super Effective Damage (TM) from Grass, at least.
Alternatively, PKMN Trainer Cheryl sometimes shows up in the cafe in the
Survival Area, though it's only a chance--and even if she does appear, you can only fight her once per day. Her team of Wobbuffet, Drifblim, Wailord, Hariyama, and Blissey is in the early 60s for levels and will give you
11 HP overall. Just make sure that whatever you lead with can beat her Wobbuffet. Blissey is particularly wonderful for experience, as well.
Attack
You have some options.
1) On
Route 203,
Youngster Dallas is the second trainer you'll spot if you're coming from Jubilife, and he builds up to a team of Luxray, Bibarel, Golem, and Staraptor, each level 50. He grants
8 Attack EVs, but avoid KOing Golem unless you want
3 Defense on the side. Having Golem around does make it easy if you're training a separate wall simultaneously, but if you're EV training via Exp. Share, then this trainer is not a viable choice to train purely the Attack stat.
2) On
Cycling Road,
Cyclist M John, the fifth cyclist down from Eterna, builds to a team of Farfetch'd, Staraptor, and Dodrio for a cool
6 Attack, all in the high level 50s. He's not quite up to par with Youngster Dallas for raw EVs, but at least one Pokemon can hit all three of his Pokemon for super-effective damage with the same attack.
3)
Swimmer M Sheltin is out and about past the shores of
Route 213, packing three Gyarados for
6 Attack. It's a bit annoying to Surf out to him and need Repels, but the Pokemon are only in their early 30s, unlike the above two options. And that 4x weakness to Electric might benefit you.
4) Another lover of the seas,
Swimmer F Jessica is the first trainer of
Route 220 coming from Sandgem, and she uses three Goldeen and a Seaking. That's
5 Attack EVs, which is less than all other options here, but her Pokemon are easier to beat than the rest as well, being as low-level as the Gyarados above but not having Intimidate. And because there are four of them, Pokemon holding a Power Bracer (adds another 4 Attack EVs on top of the EVs from whatever it is you KO) will actually get more from her than from others.
Defense
1)
Hiker Kevin stands atop a mountain on
Route 207 with a team of three Geodude and an Onix. He's tied for the game's best by giving
4 Defense, and Power Belt users will particularly love him. Low-level Pokemon with a Water or Grass attack can take him on their own, as all his Pokemon are below level 20.
2) In the post-game's Sandstorm-laden
Route 228,
PKMN Ranger F Krista has a Lairon and Marowak at level 57 each. She gives
4 Defense too, and the Sandstorm conditions negate most of the speed advantage that could have come from only having to fight two Pokemon instead of Hiker Kevin's four. However, if you need to level up while you EV train, she's a better choice.
Special Attack
1) On
Route 204,
Lass Samantha is the final trainer you'll spot coming from Jubilife--the southmost, in other words--and builds up to a team of Combee, Beautifly, and Roselia in the early 50s. Like Youngster Dallas, you can't use Exp. Share here because the Combee gives
1 Speed--unless you don't mind 1 Speed, naturally. Sweep the other two for
5 Special Attack.
2)
Route 209's
PKMN Breeder F Jennifer, who's the third trainer from Solaceon, carries an interesting team of Roserade, Clefable, Mr. Mime, and Espeon, hovering around level 50. Roserade and Espeon are the ones you want to fight for
5 Special Attack, while Clefable gives
3 HP and Mr. Mime gives
2 Special Defense. The main issue with fighting her is that only half her team is helpful to special attackers, so you're using time by switching before Clefable and Mr. Mime come out. And again, Exp. Share is out of the question here.
3) The rainy area of
Route 212 needs you to pack Cut and bike north to
Scientist Shaun, who's hiding behind a tree with his Magneton and Alakazam in the mid-50s.
5 Special Attack again, but unlike the above two, you can freely use Exp. Share. High levels for his Pokemon give good experience, too.
4) From Veilstone,
Ace Trainer F Maya stands just at the entrance to
Route 215 with a Roserade, Gardevoir, and Licklicky hovering around level
60 this time. Here you can get
6 Special Attack, which beats out others, but this is another case where Exp. Share is unviable, as Lickilicky gives
3 HP. She's the best here for experience points, though.
Special Defense
Beauty Devon on the east side of
Route 214 carries three Wormadam at level 24 and gives
6 Special Defense. Nobody else approaches her.
...Still, if you're ever really, really bored of fighting her after a while then I guess you could try
Lady Melissa in front of the Pokemon Mansion on Route 212. Avoid her Cherrim and its 2 Special Attack, but Hoppip and Bellossom combine for 4 Special Defense, and Lady Melissa does give over 18000 cash if you're carrying an Amulet Coin or Luck Incense. Her Pokemon are level 58, also.
Speed
1) On the north end of
Route 205, near Eterna,
Fisherman Andrew will rock your world with the devastating power that is six Magikarp ranging from levels 8 to 14. If you can manage to take them down, which tends to require that you be at least so high as level 4 (lest they PP stall you with their defenses), they'll give
6 Speed EVs total. Power Anklet users will truly love this guy, but if you don't have one then you should probably go elsewhere since it takes a while to knock out six Pokemon.
2) The southmost trainer of
Cycling Road,
Cyclist F Kayla, constructs an Electric mouse team with Plusle, Raichu, and Minun, each level 58. She grants
5 Speed, and overall there's little reason to train with her, but maybe she could have her uses if you're already around Cycling Road for Cyclist M John. Too bad they're not within the same VS Seeker range.
3) On the west end of
Route 211, coming from Eterna,
Ninja Boy Zach runs a level 51 bat team with two Crobat and a Golbat that must have been knocked out too much to love him and evolve. He gives
8 Speed, which is the game's best, and the only reason one might possibly hesitate to train with him is that you'll need Repels since he's out in grass. It's not so bad, though.
Money
On
Route 212,
Gentleman Jeremy and
Socialite Reina stand around and are ready to give you charitable sums of
24,400 each (if you're holding an Amulet Coin or Luck Incense) for their level 61 Chatot and Roselia.
I don't recommend trying to simultaneously EV train and make money, since it just slows down each individual process, but if you want to, then (in addition to Lady Melissa as mentioned in the Special Defense section) Socialiate Reina's Roselia gives 2 Special Attack, and Rich Boy Trey at the west end of Route 222 gives 3 Attack and almost 20,000 in cash with his level 61 Luxray.
Staryu carry them at a 5% rate, but in Platinum the Pokemon themselves are rare... I happened to get about fourteen Star Pieces just from going through the game normally and using the Dowsing Machine, but they're going to be rare from here on out.