LED's for "D" Maglite?

Digging thru my old stuff came across a couple D size 3-cell Maglites. I would like to upgrade these to LED's. Whats the best option out there for changing these out?

Have used one made by nite ize. Available at Home Depot. Heard terra lux makes a good one.
Have not tested which is best...

They make a kit not sure of the name but I recollect I had seen it on this site in the modified section

You have a few options available to you depending on what you want from your lights. The simplest option would be a PR flange LED drop-in since its as simple as changing the bulb on your lights. I have no experience with the Terralux products, but, they're recommended here often. I use the Nite Ize PR flange drop-ins in my emergency lights. They offer two models, a 1Watt and a 1/2Watt model. The less powerful half Watt model can easily be identified as it has a large LED dome, about 3/8" in diameter. This is my long term "I don't think the lights are coming back on for a week and I need to preserve my batteries for as long as possible" bulb, waiting in the tail cap. The primary bulb is their one Watt model as its considerably brighter but lasts only 2/5 as long as the half Watt one.

There are also modules that are considerably beefier than the simple drop-in bulb, like the Malkoff Devices Mag upgrade or the Fusion 36 module. These require a minimal amount of work to get up and running and will be far brighter than the simple drop-ins.

A third option would be the custom option. I like to make my own heatsinks for 3D Mags incorporating a Cree XM-L driven by a three mode 2.8A linear driver for an honest 550+OTF lumens with the option to reduce the output as needed. You don't need 550+ lumens under the sink or in tight quarters, for example. A simple on/off toggle of the power button changes modes until you find one that'll work for your situation. If even the low is too bright then you're out of luck, though.