My first two are easy:
1. GURPS 4e
2. Mutants & Masterminds (probably 4e, from the way it is shaping up)
With those two systems, the breadth of games I could run is pretty vast.
After that it becomes harder to pin down a third system. I thought about Cortex for the quick games or one-offs. But a third generic system seems like a bit much.
A game dedicated to general/generic fantasy would make a lot of sense, so maybe D&D 4e or Shadow of the Weird Wizard. I am interested in reading more on Daggerheart, but haven't yet, so I can't commit.