Probably Gascoigne in Geoff Crudgington's testimonal game (89 or 90).. Without that cruciate knee injury in 1991 he would have surely been ranked one of the best players of all time (and I mean in terms of world football). His talent was incredible- although still a good player at Euro 96 etc, he was never the player he was before that nasty injury. That was only Gazza at 75%.
I saw Messi earlier this year and was surprised at how good he was (sometimes these players fail to live up to their hype when you see them, but Messi was worth the entrance fee and really did stand out amongst a team of superstars).
Have also seen Jari Litmanen of Finland, who sticks in the mind with his skills.
I guess Paul Dalton is my Fav Argyle player fror skill levels. He's not as good as the above though !!!