I have been trying out lots of different ones.
I want my teeth to be WHITE, like Tic Tacs. Unfortunately, the natural color of my teeth is nowhere near Tic Tac white, so chances are even with professional whitening it wouldn't be that dramatic.
What I have found is that the generic Target brand gel works well. The trays it comes in are TERRIBLE! So I had a custom tray made, and I use a syringe (like you use to measure liquid meds) to suck the gel out of the crappy trays and squirt it into my custom tray.
I also have super sensitive teeth, so I can't tolerate the whitening gel every day. I have been using it about every other day.
I have been using it lately just to see what it does. I will buy another box of the trays before we go to WDW and use them the week prior.