Virtua Fighter 1 was okay and hard to pull off the good moves.  However, button smashers could still beat decent players.  If a button smasher can beat average/pro person in a fighter then that fighter isn't the most difficult.

I believe Smash Bros. is the most difficult to beat a pro at (limited items or no items).  You can't just sit there and smash the controller buttons and win.  A button smasher will never beat a pro in Smash.

Super Smash Bros. Brawl, it was originally developed with the intention of attracting the 'purple ocean' consumer (aliens), but when the localisation fell through Nintendo decided to release the game on Earth without making any enhancements for human limbs.

If your asking for a game that was designed with the intent to be hard, then Virtua Fighter 5, I can barely throw a punch without calculating 43 algorithms.

Virtua Fighter -> very technical game

Tekken -> very steep learning curve, can start off easily with an easier character like Eddy Gordo but can be very technical as well

LARS FOR THE MUTHA EFFING WIN!

from 2d fighters I guess it might be SF..but not a big fan of 2d fighters

It's Mortal Kombat for me because it has become way to hard to master. Yea you can find some moves from a character than can win you a few rounds, but its layout has become much harder for the first two games.

I was wondering when someone would say Mortal Kombat =p

Dragon ball z infinite world :O jk
Seriously its Street Fighter I just can't get into it...but then again I haven't played many fighters..

Mortal Kombat isn't too hard, but it is very annoying. It's full of insta-hits, insta-teleport moves that you must pretty much reflex to block. To make matters worse, block is a button that must be held, instead of the simple formula of Street Fighter (hold directional button on opposite direction to block).

The new Mortal Kombat made things even crazier with super buffed offense, the X-rays are super insta hits that deal absurd amounts of damage.

I find it interesting that Virtua Fighter has one of the least complex controller schemes out there, but is argued to also be the hardest fighting game out there.

competitively? ill say virtua fighter or tekken, as they both have lots of options for all characters.

street fighter IV( some of the combos let alone special and ultra combos are near impossible to on the fly with a regular controller)