Its hard to pin it down really. I have always wanted to play guitar so it is hard to narrow it down. The 3 that really got me motivated to actually go buy a guitar and try to play the dern thing:
1. Ted Nugent
2. Mark Tremonti
3. Kirk Hammett

I can't say they really influenced me as far as style of music goes. They definately made me say (as I was playing air guitar) "I wish I could do that"!