I have a number of D Maglites with various levels of upgrades Terralux TLE 6 EX, TLE 300m EX and EX S and a recent Malkoff. I also have the TLE 310 on the way.
It all depends on what you want (and want to spend). The TLE 6 EX is a simple replacement for the bulb. The beam quality will be slightly imporved (due to no filiment), somewhat brighter and still focusable. It's very simple to install, fairly cheap and run time is incredible. I been using them in several lights for a couple of years without any problems.
The TLE 300M EX is easy to install and very bright. The light is no longer focusable and projects a large flood of light, no real spot or spill. Good for medium distances. The EX-S is single mode and the EX is three mode. Run time is ok on high, I get at least 3 hours or more in a 5 cell with NiMH batteries, the multi mode can obviously go much longer. A little thermal paste where the module contacts the head and body will help with heat. I gave two of these to a couple of State Trooper friends for their Mag Chargers, after more than a year they're both still going strong. I think that says enough about durability.
I installed a Malkoff in a 4 D a couple of months ago. Very bright but not as bright as the TLE 300 overall, more of a thrower. Build quality is without question! As is, you can drop it right in simply cut the cam off the stock reflector. Light is still focusable and much brighter than stock. You can also OP the reflector yourself if you want to smooth out the beam a bit. Since I already have several floody Mags I decided to build a thrower with the Malkoff. I cut the switch down as short as possible and set the Malkoff as deep into the body as possible. I then bought a couple of deep reflectors for the Mag Rebel LED lights and cut the cams off them. I OPd one and left the other SMO. It's a tight fit but worked out fine. With the SMO reflector the beam is an incredibly tight spot, just what I was trying for.
I forgot to mention, there is a link on Malkoff site to receive an email when out of stock items become available. I clicked that, entered my email address and was notified a few short days later. I ordered right away before it went out of stock again.
I'll try adding a couple of pictures below, I have mixed results at times.