Mt Moon in Gen 1. Boring, and way too long for being so early in the game.
Rockets in Goldenrod City in Gen 2. There's like 45 fights there
Everything after Lilycove in gen 3, especially seafloor cavern.
The Mt. Coronet climb, all the way up to Giratina in Gen 4.
Chargestone Cave in BW1, Everything from the Plasma Frigate to fighting Ghetsis in BW2
>>29282062 in Gen 6