What is your favorite tank for a betta?

I like tanks with ample swimming room, good filtration without too much flow, and a heater for a betta. I kept my betta with guppies and an amano shrimp and he left them alone. It all depends on the temperament of your betta.

The 6.6 gallon bookshelf kit from Petco seems like an ideal betta tank:
http://www.buzzillions.com/reviews/aquatic-gardens-petco-bookshelf-aquarium-kit-reviews
Ample swimming room, but no need to buy a stand which can often times cost more than the tank itself.

I like those 2.5 and 5 gallon mini bows w/ built in filters for a single betta. You just need to add a small heater.

6.6 Bookshelf tanks look awesome! Gives much more room for the betta and for future inhabitants that you're gonna have.

I really like my Marineland Eclipse Hex 5 for my betta. Petco has a pretty good sale on them right now. The small footprint won't take up a ton of room on your desk, but it's a decent size at 5 gallons. Everything is built in except the heater. You have to keep the water level fairly high for the filter which causes a bit of whirlpool action at the top, except behind the output. However, because it's deep, the water is very calm in the bottom half. So, my guy likes exercising, playing and surfing at the top or just hangs out in the lower half and/or hangs out and makes his bubble nests in the calm behind the output.

I'm with ExtraHannah. My betta appears to love his Marineland Eclipse Hex 5. I had him in a Eclipse 3, but when I converted that to a shrimp tank, I bought him the hex 5. He seems to be much livelier in the Hex 5 (or as lively as a betta can or will be).

that Fluval Edge looks cool......

Well, I like the bigger the better for tank sizes but for a desk I'd say a 5 gallon.