Superb flank opening in your opinion and why?

Read the thread before answering! Catalan opening starts with d4 it's not a flank opening, so as modern defense

I tend to use English and Bird to spice things up from time to time. But don't take my word on openings, as I've been a frequent user of the Latvian Gambit.

My bet would be on the English Opening since I have more control over the pawn structure than black does.

I like the King's Indian Attack (also called the Barcza System).
White has tremendous flexibility in arranging his center Pawns... he can go for an e4,f4 duo, and/or a c2,d3.e4 chain, or play c3 and d4 (up beside his e4 Pawn), or push on from e4 to e5...
All of those central formations are fully playable in the KIA, so you can choose the one that best opposes the set-up that Black has chosen.
Here's a game in that line by an American high-school drop-out... Bobby Fischer.
http://www.chessgames.com/perl/chessgame?gid=1044267

Briefly flirted with the extremely flanky Benko Counter Gambit against 1.d4 when I was younger and gave a few of my opponents both a nasty shock and a bloody nose when I played it, but didn't enjoy the games as much as I did playing the Grunfeld or Kings Indian Defense, particularly when the players of the white pieces facing the Benko knew what they were doing. One of my team mates on the Nicaragua chess.com team, fredd-afrack, has had some impressive victories with it as black.
In a classic game(not blitz).
Superb = best, highest chances on winning, etc..