Pikachu doesn't require any explanation.
Charizard, also no explanation needed.
Snorlax is often seen as Ash' third strongest Pokemon, after Charizard and Pikachu. Snorlax has done well in the Jotho League, and in Ash' gym battle against Clair.
Bayleef, Ash' most underrated and unappreciated Pokemon. Bayleef has been defeated only twice since its evolution, in the episode "Current Events". Both defeats were in the Jotho League, against Gary and Harrison. That means its raw achievements and win-loss-ratio are much better than any other of Ash' Pokemon. Statistically, Bayleef is Ash' strongest Pokemon. I know Bayleef isn't, but she definitely deserves to be on Ash' team of strongest Pokemon, without a doubt. Bayleef did in some important battles: as a Chikorita, against Bugsy, he single-handily won Ash a Storm Badge against Chuck and did an awesome job by defeating Harrison's Houndoom in the Jotho League. Bayleef is the first Pokemon I thought of when seeing the topic title.
Sceptile has done well in many battles, in the Battle Frontier. It is, in my opinion, stronger than Ash' Bulbasaur (but Ash' Bayleef is at least just as good as Sceptile).
That leaves one spot open, which could be filled by Muk, Tauros, Kingler, Donphan or Heracross.
I have to admit that I hate Gen IV and that I haven't even watched a single Gen V episode (and I don't want to). I'm biased, being a Gen I and II anime addict.