I'll concur that its difficult to decide. When particular artists stand out in my mind its for different reasons based usually on three different factors:
1. Style, quality, coloring quality, quality of lineart, skill, talent, experience.
2. Imagination, ability to make imaginative things seem real in this world and not contrived.
3. Originality, consistancy.
Since this is about tickling however, I've always said its a tie between Scamwich and Osaka simply because the quality of the lineart and the coloring job is above par. The entire package is impressive with their works, whether its fanart or original creations.
Honorable mentions include Zero, Slaanesh and now Nessonite. I think its fair to say Ness has come along a great deal in a short ammount of time for the genre she chooses to draw for/from. Thats impressive no matter where you are and this isn't the first time I've said this.
Slaanesh has very good lineart and sketches. He used to be my rival in the sense that we are both good, we are both consistant, we both deliver, or at least try to, and we both have alot more to do yet.
And Zero, he's been very consistant. He has his highs and lows but he doesn't give up. His lineart is also good and thus worthy of recognition.
@ AphxA- Thanks for the mention there. ^^