My favorite fish in my own tank is a Pelicier's Perchlet. He has a personality a lot like a Flame Hawkfish, but less shy and bigger. He's always photobombing.
My most desired fish right now is a Candy Basslet, but I keep getting distracted with more rare fish and spend my budget there instead...
If I could only have one fish it would be a Randal's Goby with a Pistol Shrimp because one fish would likely be a nano and these are the most awesome nano fish imo. If I could only have one type of fish it would be Anthias because a whole whack of Anthias are just beautifully amazing.