-Charizard
-Sceptile
-Bulbasaur
-Squirtle
-Swellow
Charizard Ash's strongest pokemon from OS-BW and currently still his strongest
base pokemon without relying on mega evolution,abilities,items etc.It got the most legitimate training out of all his pokemon,it's the equivalence of the Z fighters from Dragon Ball training in the hyperbolic time chamber and coming out much stronger than ever before.From OS/Johto to BW/Unova Charizard was constantly training at the Charicific Valley.It was getting stronger,learning new moves and bailing Ash out of tough situations (Clair) (Gary) (Noland).I honestly think that Ash could have won long before Sun & Moon if he had used Charizard in some of those pokemon leagues after Johto.
Sceptile Ash's strongest grass-type pokemon,I always liked it ever since it was a Treecko,cool,calm and collected.It was one of Ash's strongest during his Hoenn journey then took the #1 spot when it evolved into Sceptile,it's performance against Frontier Brain Spencer sealed the deal.The last great feat it pulled off was taking out Tobias's Darkrai who was undefeated in battle.
Bulbasaur Ash's second strongest grass-type pokemon,it's very mature and has been known to show leadership.One of Ash's best pokemon from OS.It has a knack for defeating a pokemon and then drawing with the next one in battle (Jackson) (Brandon).
Squirtle one of the few pokemon I liked from Ash's party in OS/Kanto,it has a pride as a Squrtle that it can be just as strong and dependable in situations like fighting fires like a Warturtle can.It's the leader of the Squirtle Squad and has it's own pair of sunglasses which are different from the rest of the Squirtle in the squad.In battle I'd say it's Ash most skilled water-type,constantly fighting fires gave it the ability to see through smoke and hit it's target and have deadeye accuracy even while in mid-air.
Swellow Ash's strongest
bird/flying type,it's a very stubborn pokemon in battle and was evident in it's debut appearance in early AG,it took Ash forever to catch it.It was one of Ash's best pokemon during AG/Hoenn and had a case for being Ash's #1 option at the Hoenn League which is something you can't say for the rest of Ash's bird flying types.