Sparky - As a medium weight Electric element Arm, Sparky is arguably the best Arm in the game. Keep it charged and lead off with this attack to shock the opponent and allow for any number of follow-up attacks. It will beat all Light attacks and is fast enough so it doesn't leave you open to a counter attack very long if you happen to miss.
Thunderbird / Phoenix - Another Electric Arm, the Thunderbird isn't as fast as Sparky, but it has much better curve ability and it can catch opponents off guard due to the somewhat delayed attack of the bird connected to the Arm. This also goes for the Phoenix if you already have an Electric Arm and want more damage to go with your curve. Although in that instance the Slapamander is usually a better choice.
Slapamander / Slamamander - One of the most annoying Arms in the game is the Slapamander. It's difficult for opponents to get away because it has a good curve arc and a large hit box (area of attack). You usually want the Slapamander over the Slamanader for the extra damage when charged, but both can be effective.
Megaton / Megawatt - There are going to be times when you're constantly trading with an opponent, both Arms canceling each other out. That's where the Megaton comes into play. As a heavy Arm it grounds lighter Arms as it continues toward your target. While it moves slower than most Arms, if used to counter an opponent's lighter Arm it almost doesn't matter how fast it moves so long as you can react fast enough when you see the opponent punch. A charged Megaton is also one of the most damaging attacks in the game, making it a great follow-up after a shock. If you find that you're leading off with the Megaton, switch to the Megawatt to shock an opponent so you can get a follow-up throw.
Cracker / Popper - The advantage of the Cracker and Popper are speed and a wide hit box (area of attack). As Light Arms they travel fast, but with two attacks it will take a very well-timed Medium or Heavy Arm to stop both hits of the Cracker and Popper. Couple this with the wide hit box and speed, and it's difficult for many opponents to avoid a well-timed attack. We generally favor the Cracker over the Popper because you're already giving up damage for speed if you don't connect with both hits. Cracker at least gives you more damage if you hit a charged attack. You can occasionally follow a charged Popper with a second attack, but the timing can be tricky. If that's the route you want to go, the Parasol is a better bet. It's a bit slower, but the wide hit box is similar to the Popper.
Blorb - While the Blorb is very similar to the Megaton and Megawatt, and works well if your opponent doesn't have a Heavy Arm to counter your attacks, you sacrifice damage in exchange for a bit more mobility (it bounces) and the ability to blind an opponent. While the blind is nice in theory, a blinded opponent can still block, dodge and attack just like normal. This makes the Blorb far less useful against skilled players.
Boomerang / Chakram / Coolerang / Ramram - These high curve Arms work very well to attack around obstacles in the various stages. They work especially well to attack from behind the columns in the Scrapyard and DNA Lab, as well as the cars in the Cinema Deux stage. However you don't gain much benefit beyond the high curving ability, and most skilled players will either block or evade if they think you might attack from behind an obstacle (or while they're hiding behind an obstacle). Even if you land such an attack, it can be difficult to follow it up with additional damage because the obstacle is still in the way.
Bubb / Buff / Chilla / Toaster - All of the common boxing gloves in the game are almost identical to the Sparky. What makes the Sparky so much better is the fact that it shocks an opponent and allows you to follow with an easy throw or some other attack. Buffa has a bit better curving ability, Chilla slows down an opponent's movements, and Toaster inflicts more damage, but none of that outweighs the shock power of Sparky. However, if you're leading off with a different Electric element Arm, Toaster is a good secondary Arm since it's fast and inflicts very good damage.
Dragon / Ice Dragon - The two dragon Arms are very unique. The fire a beam that goes through some Arms, but can be stopped by others. You also get a good attack radius with the beam that covers a decent amount of ground and works very well against opponents that are trying to keep their distance. Where the dragons fail is in close combat. If a skilled opponent closes in on you, the dragon Arms are very difficult to use effectively and usually end up leaving you open to a counter attack more often than not. Ideally you want to use the Dragon as a secondary Arm to follow an Electric shock for good damage.
Guardian - One of the more unique Arms in the game, the Guardian is basically a shield. The initial attack is a shield that stops everything, no matter the weight. You can hit the attack button again and the Guardian will extend farther out at a faster pace in an effort to hit the opponent. Even after the shield portion of the attack, the Guardian is still one of the slowest Arms in the game. This means it should be used primarily as a shield, and if it happens to hit an opponent all the better. This works well against Heavy Arms, but is easily bested by Arms with high curving ability or an opponent who attacks from close range.
Homie / Seekie - These two Arms will move toward an opponent no matter how you curve it. This can come in handy if your aim isn't very good. In addition, a charged Homie or Seekie will hit like normal, then explode after a short delay. This can easily catch an opponent off guard, especially if they're playing a highly mobile character like Twintelle or Ribbon Girl. These characters will assume they've dodge the initial attack, then potentially get hit by the delayed second attack. Even with these benefits, they are relatively slow and difficult to hit skilled players with.
Hydra - If you're playing against an opponent who likes to jump a lot, the Hydra is a great answer. It fires three vertical shots that can be difficult to evade in the air. However, on the ground all three shots are fairly easy to evade. They are light attacks, which means they will get beaten out by most other Arms, but due to the fact that there are three shots in vertical alignment, it's rare for most Arms to hit all three shots. Unfortunately, even if the last shots sneaks in, the damage on a single shot is a bit low (80).
Retorcher / Revolver - These Arms fight three shots in rapid succession. While all three shots are light, they can be difficult to evade when timed correctly. While the Revolver is an Electric Arm, the stun is negated if more than one shot hits. Unfortunately this makes it one of the worst Electric Arms in the game. The Retorcher is the way to go if you like this Arm style.
Parasol - The main advantage of the Parasol is the wide hit box that makes it difficult to dodge and can partially block an opponent's vision as it travels. It's still a Medium Arm, which means any Heavy Arms will knock it to the ground and continue toward you. If you don't need the larger hit box it's usually better to go with a boxing glove or even a Heavy Arm to counter Medium weight Arms. If you like the wide hit box but want a faster Arm, the Popper and Cracker are better.
Triblast / Tribolt - These Arms are similar to the Hydra except that they shoot three shots in horizontal alignment, making them difficult to evade. Triblast is generally the way to go because of the extra damage, but as an Explosive Arm the charged damage is delayed in a second hit that can technically be evaded, but will usually catch an opponent off guard. When used as a secondary attack the delayed follow-up usually misses when using Tribolt, however Triblast becomes a single attack when charged, allowing it to connect better as a follow-up attack.
Whammer - The Whammer has a wide hit box (area of attack), which can make it difficult to evade once it slams to the ground. Unfortunately, it has to travel to the opponent before the slam occurs, and that travel speed is relatively slow. It's a Heavy Arm, which means it goes right through all lighter Arms, and that's how it should mainly be used. However, even in this instance it can be difficult to hit more mobile characters.