There are many strong Pokemon on his team. If we are going by evolution (which the anime doesn't really) Krokodile, Unfezant, and Leavanny would be his strongest Pokemon.
If we go by how often they win battles, well, this is Ash we're talking about which makes them all strong.
Just some facts and theories on the Pokemon below.
His Scraggy is not as powerful as others. It's move is described as "not powerful" by Serebii.
Palpitoad is said to be quite strong with a STAB Hydro Pump.
Pignite with it's three STAB fire moves is one of his top Pokemon.
Snivy in Ash's hands seems to have power, unbecoming to it's small size.
His Boldore also is very strong managing to "Hold its own" against Clay's Excadrill.
Oshawott has in the anime won the most battles for Ash so far however.
So I think the top 5 possible strongest Pokemon on his team (excluding Pikachu and Charizard) are:
Krokodile
Unfezant
Leavanny
Oshawott
Pignite
not in order
Source A
Source B
