Hmm, this is a tough one. I prefer upperbody to feet, but beyond that all spots have their own charm.
-When underarms are stretched and restrained, its great to tease down their arms, and slowly build up the tickling under her arms. Pulled taut, to able to take your time reducing her to a giggling mess... Unrestrained, its more wriggling under clamped arms which is less fun.
-Belly tickling is wonderful: its the part that quivers and reacts to tickling and laughter anyway. To stroke over the quivering flesh, exploring the expanse of soft skin for sensitive spots... plus a smooth, toned belly on a girl is something I find particularly stimulating. Discovering a ticklish belly with a few hard to find hot spots is like stumbling upon a treasure trove.
-Breasts are great for obvious reasons. They're already wonderful even before they're ticklish. Gliding over the undersides or where the breasts meet her sides provoke great reactions, and mixing the tickling with arousal is always a plus. I've never done any cooch tickling, although the idea is intriguing.
So I can't decide! I guess the ideal would be a beautiful, nude woman wanting nothing more for me to tickle them silly. Trying her hardest not to bad my hands away as I wander over every inch of her, finally restraining her wrists and stretching her out deliciously. Tickling her until she decides we should do other things...