It's definitely a toss up for me. I think at this point I've seen The Melancholy of Haruhi Suzumiya more than any other series. It's pretty much my go to series whenever I'm down. It has it's issues, but I really like the characters
I've also watched both Boogiepop series and Bloom Into You many times as well
edit - adding D-Frag as well for feel good series I watch almost once a year
I haven't seen those Gundam OVAs! I'll have to check them out. Gundam Wing was one of the first anime I got into when I was growing up (along with the rest of the Toonami block)