Depends on how much you're willing to spend. The whole "approved" thing isn't a big deal either. For the money, the $19 MagLED dropin is pretty good. It's pretty bright when you first turn it on, but drops to 50% output after being on for a few minutes because they designed it that way for heat protection.
The next best bang for the buck dropin is the Terralux MiniStar5:
http://www.batteryjunction.com/tle-6k2.html
It's the brightest inexpensive dropin available for 3-6 cell Maglites.
Currently the ultimate dropin for 2-3 cell Maglites is the custom made one from Malkoff Devices:
http://www.malkoffdevices.com/
It'll cost you $65 (+ $5 for a camless reflector if you don't want to cut yours) and he's on a production schedule, but this is hands down the best dropin that can be bought here in the US.
You could buy the Gordon, but it's not a Mag. Since you've already got the flashlight, you mine as well upgrade it.