Damn, I wish I would have known that. I spent soooooooo much time trying to get her yesterday. So who's the best character to get starting out? My hopes got dashed on the ground, so all that's left is to pick up the pieces.
It really depends. Short term (as in NA right now) or long term (based on JP releases)? Also depends on whether you value early game, as some characters only become really good (Vlad for instance) after you leveled and ascended a few times, while others (Jeanne being one) work great early game and fall off a little late game.
Mostly, if you are rerolling, pick the one character you really really like in the franchise since you can guarantee yourself getting him. You'll get other 4* and 5* eventually, just that they may not be the ones you are looking for (E.g a lot of people got Atlanta while trying to get Gilgamesh). I lucked out into Altera, and even if she isn't seen as a good 5*, she was hitting as hard as Herc and much less squishy.
Power level wise (obtainable by rerolling now, best to worst, mostly from thread consensus/friend support usage since I don't have most of them):
5* (Frankly all the 5*s are good, and better than just about all the 4*s, even if some are better than others)
- Waver (Caster) : Best support in the game right now, and still one of the best in JP. Works well with everybody else, can buff NP by 50% to a single person and 20% to the rest of the team.
- Vlad (Berserker): Gets good in mid game (Lvl 60+). Great damage dealer for Arts-based teams. Berserkers are somewhat squishy but great for farming.
- Jeanne (Ruler): Just doesn't die, resists everything but Berserkers. Really great early game. Since she doesn't die, you have a good chance of completing most quests even if she has to solo it. It might just take forever since her damage is on the low side and her NP doesn't do damage but saves your team. Also really easy to level and by far the cutest =)
- Altera & Arturia/Altria (Saber): Nice stats and AOE damage. Excalibur hits harder, but Altera has a DEF debuff. Also good DPS Sabers are really rare currently, so point in their favour. Apparently, they get outclassed by future characters.
4*
- Herc (Berserker): Best 4* char and you can get him in the guaranteed roll. Does good single target damage. Like all Berserkers, somewhat squishy, but after a long long long time (Bond 10), becomes undying just like in the anime. Actually he does die just like in the anime, just that it takes a lot of effort.
- Saber Alter (Saber) Dark Excalibur hits harder than regular Excalibur at equal levels. Generally plays like a frailer but harder hitting FSN Saber. Story-locked to Fuyuki, so really rare.
- Nero (Saber) Arts AOE which is pretty rare. 2 Heals and a future skill that gives multiple Guts procs means she becomes a one woman army. Story-locked to Septem, so really rare, though many ppl rolled for her at Nerofest
- EMIYA (Archer): Has 3 Arts cards, rare for an Archer, so can cast his NP often. Stats are kind of weak till late game (60/70+), and Unlimited Blade Works doesn't do that much damage. He gets a lot of buffs in the future to make him much much better. Also featured in many events in the future, so having him makes your farming easier.
- Lancelot (Berserker): AOE berserker that needs support to help him charge his NP. Once charged starts wiping out waves. Can't be gotten in the guranteed roll.
- Everybody else but Stheno: All have niches in which they excel. For certain classes (Lancer, Rider, Assassin), these can be the best ones available for a while (thanks to Ala Alba for mentioning)
- Stheno: Kind of sucks since NP does no damage, gets a few buffs far away in the future to become an ok char paired with Divine characters