I've made my own submersible 4' fluorescent light and love the light output on that.
I like the 4' lights because they illuminate in 360 degrees and have a high brightness output (lumens). The downside to the 4' lights are that they're large and a little harder to store and they consume a lot of battery power. You would be well advised to have a spare battery for one of these lights... or have a generator on board like a few of the club members do here.
In all reality, the best lights are the ones that you can afford. I've used those cheap 1 foot lights you get at Walmart and they didn't work at all. Like Hawk said, the brighter they are, the better!