I am just like
@AshleyC in this regard - I can be made to giggle just by someone pointing a wiggling finger at me (definitely more so if the finger is approaching).
As regards 'that spot', I have way too many. Underarms, just like Ashley said, is one of my most hyper-ticklish spots - simply touching a finger there makes me melt with laughter, no need to even move it. Then there is my lower belly - even if I myself touch a finger-tip there, it involuntarily sucks itself in suddenly. And then there is the crease where the thighs meet the belly, which I feel is like the armpits of my legs - deathly ticklish. Again, the area just around the hipbones is crazy ticklish for me - it will have me jerking and thrashing around like a fish out of water. Feet, especially the centers of my soles and under my toes, drive me nuts. And if you really know how to tickle well, my ribs, butt, tailbone, backs of knees, and inner thighs are also very weak spots, ticklishly speaking.