Here are some of the best I remember from watching more than 90% of all the anime episodes.
Indigo League
Metapod vs. Metapod (for the comedic aspect)
Ash vs. Jeanette (Muk being the ultmate troll)
Orange Islands
Ash vs. Cissy
Ash vs. Drake
Johto League
Ash vs. Falkner
Ash vs. Gary
Ash vs. Harrison
But overall, Johto sucked. Even through my nostalgia glasses I still lose interest. It's the only series where I can't remember what happened in the majority of episodes.
Hoenn League
Ash vs. Tate and Liza
Ash vs. Juan
Ash vs. Tyson
Battle Frontier
Charizard vs. Articuno
Sceptile vs. Blaziken
Sinnoh League
A lot of contest battles because of the CGI graphics, especially Wallace Cup and the Grand Festival
Ash vs. Paul, Lake Acuity
Ash vs. Nando
Ash vs. Conway
Ash vs. Paul
Ash vs. Tobias
This was the best anime series. Ash being competent, contests done right, lots of characters, interesting landscape, etc.
Unova League
A few club battles but even that's a stretch.
Decolore Islands
Why does this even exist?
Kalos League
You watch it for the beauty that is the Kalos region, not because Ash is any good.