The guys who you can't resist adding to your team every run, every game (unless you're running a specific lineup that they don't fit), and why.
Zubat appears in just about every game really early on and I get a kick out of catching something at like level 3 and still having it in my party when I beat the E4, also dat speed.
Vaporeon is aesthetically cool and is great at soaking up damage while dishing it out too.