A Green Spotted Puffer! They're just very different fish from what I've had. I've never had brackish fish or a fish with so much personality as I hear puffers have. I'd go for any puffer actually, but green spotted are the prettiest I've seen so they're my official answer, haha. That may simply be because they're the only puffer I've seen in person though.
The fish I always wanted to keep, I actually have, Red Terror (Festae). All The research indicated, too big, too mean and keep in a species only tank. They don't. They are not, and I keep a small group of them in a large aggressive community tank. Glad I took the chance.
From freshwater, I've started getting into koi recently and it's crazy how expensive some of the varieties are! I'm not sure which varieties are my favorite off the top of my head but there's some really pretty kinds that I would love to have.
I really had my heart set on discus for my 125gal planted tank. But, I decided I was too lazy, my water is too hard, my pH is too high, and the fish are too expensive for me to take the gamble on trying them.
Is there a particular species you've always wanted to keep?