When I replay older games it's usually gen 1 and 2. Occasionally, I also replay gen 3 games (Emerald usually). Gen 1-2 are just very quick to get to the gameplay. Gen 3, Emerald especially, end up being somewhat slower (all these people randomly showing up to give you their number <_<) but still fairly quick.
On the other hand I just can't find myself going back to gens 4 and 5 due to how slow they feel at times and generally I don't really have access to a DS/3DS, anymore, as well.