Are we limited to X-Men characters? If not, I'd say probably the ability to read minds / influence minds. That'd be a pretty useful tool. You'd be able to influence your boss' thought process or find out what your coworkers really think of you, find out what's on your significant other's mind, or even if someone is lying to you, etc.
Pretty useful. 
I don't know if there is a superpower that doesn't exist is the X-Men universe if you include all the obscure characters. But I have to point out that the ability you chose belongs to the guy who puts the X in X-men, Dr. Xavier.
Hmm, I didn't know Prof. X had the ability to influence.. only read. But, I was thinking more along the lines of Matt Parkman from Heroes. 
Pretty sure I remember some of the cartoons where he would put thoughts/visions into people's minds but being the good guy that he is I doubt he harnesses that to it full potential.
You... you... have it.
lmao
How about the power.... To move you...
A can of chili can do that
Thats telekinesis Kyle.....
Time travel is a def, hmmmm....
I need to think about this a bit more.
The power to kill a yak from 200 yards away with mind bullets...
The simplest is always the best, the power to fly
I love Wolverine's healing power. The skeleton bonded with adamantium would just be a bonus.
I have tha powah!
Squirrel Girl... nuff said!
fOx
I had a car that ran on squirrel power.
Close enough... I'll take it.
fOx
Big_T
Like the character Hiro on Hero .. stopping time or time travel would be excellent power to have.