It's a toss-up ... and probably neither really qualified as a "gig", because I only sat in on 3 songs or so.
One was my first time ever playing with live musicians, at a drummer's hang, not having a clue whether I could do it or not. I was scared to death, but when I hit the first beat (exactly when I was supposed to!), I went from wanting to be
a drummer to being
a drummer. It was the biggest rush I've ever had.
The other was being asked to sit in with my girlfriend's band ... they're all awesome musicians, and it was a thrill to play with them ... and especially to play with my friend on keyboards. It was a gas.
The only true "gig" I've done was my least favorite. It was a disaster. Half the rest of the band stunk. Bad.