I must echo the sentiments of many posters preceding me: How can you choose just one?!
That said, my favorite toy to be used on me is a toss-up between chained clover clamps and an inflatable/vibrating plug or dildo. I can only describe the clamps (wherever my Domme decides to apply them) as "exquisite", and between the sensation and vague humiliation of a plug or dildo, it gets me deep into subspace pretty quick.
To use on someone else: I haven't had too much experience topping, but I really, REALLY like crops. A good leather crop can gently caress or cruelly spank... you can slide it over your sub's skin, relaxing them and taking them off guard for when you decide to strike. You can make lots of tiny quick strikes or hard, staccato slaps. After that, I love the aesthetic quality of a woman gagged and blindfolded--there's something intimately gorgeous about the vulnerability, and it makes sensation play even more fun. Oh, also, ice and hot wax are way up on the list.