If this is a 6' tank or a 4' tank (I am going to assume a 4')? Do you have a canopy and want a retrofit or would you rather have a fixture?
If you are looking at a 4' with a fixture, IMO ATIs (reefgeek.com) at this moment are hard to beat but they are pricey. They are kicking out some great PAR though. The SolarFlare listed above is great as well. Tek Light fixture isn't bad for the money either.
Retros, I am a big fan of the IceCaps that are overdriving the bulbs. I bought mine from ReefGeek.com as they have complete T5 kits all the way up to 60".