From the battles, it is TECHNICALLY bulbasaur with 31, just passing out Sceptile's 30 battles and Corphish's 29 battles
Youtuber "Zactoshi" made a video showing all of the win rates for Ash's Pokemon and Bulbasaur participated in the most battles, that being 31. (It's shown Corphish participated in 28 battles but when Zac made another video for Ash's Journeys team, he updated Corphish where he got one battle against Misty's politoed.... and lost) but the thing is, Misty used Bulbasaur for 5 of the battles, and the question is which Pokemon "ASH" used the most, so in this case, it is Sceptile where all of his 30 battles were used by Ash.
Source: https://docs.google.com/spreadsheets/d/1fyBG1xkbbX73RoVgzPh1AbuBJXCkRdNyqWETDq7LP8A/edit?gid=1550011343#gid=1550011343
Here are all of Sceptile's battles and the results whether it's from Treecko, Grovyle or Sceptile
WINS
1: Franklin's Yanma and Ariados
2: Brawly's Machop
3: Brawly's Hariyama
4: Nicholai's Marshtomp
5: Random trainer's Golduck
6: Offscreen battle
7: Norman's Slaking
8: Winona's Altaria
9: Rocky's Quagsire and Walrein
10: Clark's Quillava
11: Katie's Walrein
12: Morrison's Steelix
13: Spencer's Shiftry
14: Spencer's Claydol
15: Luna's Scizor
16: Tobias' Darkrai
LOSSES
1: Roxxanne's Geodude
2: Brawly's Hariyama
3: Natasha's Grovyle
4: Flannery's Torkoal
5: Winona's Swellow
6: Drake's Altaria
7: Juan's Luvdisc
8: Clark's Charizard
9: Morrisson's Gligar
10: Tyson's Metagross
11: Greta's Hariyama
12: Brandon's Regirock
13: Tobias' Latios
TIES
1: May's Blaziken
Win rate: 55%