While I agree 'pros' are pretty sick (I played the Japanese champ in the arcade in Tekken Tag...that was mind blowing - I was one of the best at that arcade (top 3) and he crushed us all with a character that we chose for him), humans do make mistakes. Granted, the pro I played made one or two mistakes every 5 rounds or so, but the AI in a game can be programmed to never error and to anticipate moves.
Try Dead or Alive 4 for example...the boss anticipates before you can get the action out (reads the input and reacts before the move comes out). There are other games that are similar, but its case by case. I can beat Tekken 3, tag and 5 on the hardest setting (whether its ultra hard or whatever it is in that game), but I had trouble on Dead or Alive on Normal. It's all up to the programing. Some characters are cheap.
That said, there are usually exploits in every game (seth in SF4 was reasonably hard until the exploit is found...then he is a cakewalk). Either way, the point is that its case by case.
As for the hardest games...I would agree that RTS would be up there...they are very complicated when you get down to it, and you need pretty good decision making power and reflexes to play effectively (maybe that's why Koreans, who are pretty hardcore at gaming love starcraft over many other games?)
Personally, I love beat 'em ups and hack n' slash (ala Demon's Souls), but I don't think they are hard. I just enjoy them.
See how funny it is, when your kid ain't got no father, played a little too much, now you're afloat in the harbor - NAS
I'd say fighters are definitely one of the harder genres, as it relies on actual skill far more often than most other genres, don't know how to play properly and you'll get absolutely murdered, and even if you do know what you're doing there are gaps in skill between players that you'll never overcome, it's not just a test of the mind, it's a full on test of concentration, reflexes, and ability to memorise (combos), some people have that skill, others just don't.
Other notable genres are the action genre (Devil May Cry, Ninja Gaiden, ect), those on the highest difficulties are pretty brutal.
Currently Playing: Pokemon White (DS), Blazblue Continuum Shift (PS3), Okami (PS2)
If so, fighting games are generally pretty easy because the AI generally doesn't approximate a real human well. They might be hard if you try to play "normally" (since they often react instantly to your moves), but you can usually just turtle and wait for them to leave themselves open regardless of the AI level.
If we're talking about multiplayer, then any competitive genre tends to be equally hard. If you're going up against the best players in any of Fighter, RTS, Puzzle, FPS, Racing, Sports, etc, it will always be very difficult.
In terms of single player difficulty, the hardest games are almost undeniably Shmups. They're the one genre that has gotten consistently harder since their inception, so it's now reaching a point of sheer ridiculousness.
http://terosclassicgaming.blogspot.com/ - Watch me beat "NES DuckTales" http://www.backloggery.com/tero - My backloggery