I usually always have my starter preset.
R/B- Typically Charmander, but occasionally Squirtle.
G/S/C- Chikorita. ALWAYS. <3
R/S/E- Mainly Torchic.
D/P/Pt- I don't play Gen IV very much, but Piplup.
B/W- I've only played through once, but Snivy was useful.
As for the others, I usually try to find an Ice type somehow because they're good against Lance in Gen II.
Gen I's pretty straight forward. I usually pick up a Pikachu in Viridian Forest, and a Ghost type, and the Lapras you recieve in Silph Co.
Gen III's team usually has an Altaria somewhere--I love Dragon types. :3
And ALWAYS have a good Water and Flying type to learn Surf and Fly, respectively.
Oh, and get Mienfoo in Gen V. You'll thank me later.