Yeah, that's a sweet flagtail. How old is it? I can't wait for mine to be that big.
Even though I just got him this week, my oscar has pretty much instantly become my favorite fish. He has so much personality. As far as a fish that's worth the most, that would be my flagtail. Around here, I've only seen them for sale once, and it just happened to be during a big sale, so I got a fish marked for $50 (for a little 2.5" fish) for like $16. Once he's a monster (so far he's around 4"), he'll be worth a couple hundred.