Pikachu - Around level 80. Ash uses Pikachu often and as Pika can wipe out Rock and Steel types pretty easily, I'd say it'd have to be one strong rat.
Charizard - Around level 80. Charizard has really increased its power due to training in the Charicific Valley, and its battle with Articuno was a testament to that.
Sceptile - Technically, to evolve, it'd have to be lv. 36(?), so, I'll say around level 40. Sceptile is a strong powerhouse on Ash's current team.
Swellow - Around level 30. Swellow is a powerful bird with a lot of stamina, but it still has some problems with manuevering.
Donphan - Around level 30. Still not the strength it could be.
Aipom - Around level 10. Aipom doesn't know many attacks and gets knocked out easily.
Corphish - Around level 25. Corphish has improved on its attack and speed since Ash met it. I'm hoping that it will evolve soon(and thus give Ash another dark type).
Torkoal - Level 20. Ash doesn't use Torkoal enough, so it hasn't gained much battle experience.
Old(er) teams:
Tauros - Level 50(or about thereof). Ash's Tauros were all pretty strong to begin with, and even though he has not actively trained them, they proved themselves in the Orange Island area.
Totodile - Level 20. Still pretty weak when it comes to taking blows.
Cyndaquil - Level 20. See Totodile.
Bayleef - Level 30. Bayleef has good defense.
Heracross - Level 40. Good speed and accuracy. Although, if I recall correctly, it knows 'Megahorn', and that's not a move learned until lvl 50(?).
Bulbasaur - Level 50. Bulba is about as powerful as it can get without evolving.