I purchased a 1w bayonet led @ frys that worked quite well on a 8.4V 2D
Can you explain what batteries you are planning to use with your Maglite? A 2D Maglite is 2.4V with NiMH or 3V dropping to 2V with alkaline, NOT 8.4V.
malkoffdevices have the best drop-ins for Maglites.
Comes in 6 versions: 2D, 3-6D, 2C, 3-6D; XML for 3-4D, XML for 3-4C (the last 2 are the brightest).
Buy the offered camless reflector with it unless you are sure you can cut the cam off your original reflector properly.
He's got a July sale (40% off)
There are brighter but you are way into custom territory.
For smaller Maglites just buy the Minimaglite Pro (1 mode) or Pro+ (2 modes).
EDIT
For just an LED bulb:
led-replacement and batteryjunction have:
TerraLUX TLE-6EXB LED upgrade for 2-3 C/D cell Maglites
TerraLUX TLE-6EX LED upgrade for 4-6 C/D cell Maglites
140 bulb lumens (~100 out the front)
superbrightleds have:
PR2-1WHP-9V 100 bulb lumens
PR2-3WHP-3V for 2C or 2D 180 bulb lumens
PR2-3WHP-9V for 3-6C or 3-6D 180 bulb lumens
The 3WHP is warmer and brighter than the 1WHP but can not be focussed as tightly.