Exciting answer: when I drew the last page of my first comic, december last year. It was 3am and I was calling with my partner on skype because I was worried I'd feel terrible after finishing my first major project. But instead of feeling terrible, I started laughing and laughed for like...5 or 10 minutes straight. It was a little scary but mostly wonderful; I felt very proud that I'd managed to finish my comic, and very excited to make a bunch more comics in the future, now that I knew for certain I could finish one.
Less exciting but more true answer: pretty much any time I get to spend with my aforementioned partner. At the risk of getting too mushy, I just feel very lucky I get to spend time with a person as great as them?? GONNA STOP HERE BEFORE I GET TOO GAY. Anyway, I prefer and tend to recall better, extended low-key periods of contentment as opposed to big giant moments of joy, so.