I've been looking for the exact same thing.
There is a 12W from Australia that is probably the brightest thing available right now (as of March 19, 2011):
Superlight Retro 12
Looks promising...unfortunately, no candela or lumen rating is given, that I could find. ) :
Cree has launched a challenge to the world to build the greatest MR-16 replacement using their XLamp MTG LED, specially designed for MR-16 applications:
Cree MT-G LED Website
Taking on this challenge, and incorporating the MTG LED, LEDz World has come up with a 450 lumen, 6.5W MR-16 replacement:
Ledzworld Website
If you find anything brighter actually available (that also fits the ANSI C78.24-2001.12 envelope and meets the Energy Star-compliant L70 rating) you let me know! ( :
EarthLED also has some very decent, dimmable MR-16 replacements:
EarthLED Pro Dimmable MR16
If you want to try to create your own MR-16 Replacement, you'll need the tech specs for the MTG from CREE (manufacturers of the brightest LEDs in the world, bar-none) found here:
CREE XLamp MT-G LED Tech Specs
as well as coming up with a driver solution (some are found here):
Diodes, Inc.
and housing (some ideas and a housing design created by Neng Tyi Precision Industries Co., Ltd found here):
Cree XLamp MTG LED MR16 Reference Design
There are other very bright LEDs available that also might be used in an MR-16 application (like Crees latest XLamp XM-L LED), but the MTG was specifically designed with MR-16 applications in mind.
My ideal MR-16 replacement would have a 50º+ angle (I don't care about spot, only wide flood), 3000-4200K, 450+ lumen beam, last 30,000+ hours, use less than 15 Watts, be compatible with both magnetic and electronic transformers, and fully dimmable (while remaining at over 120 Hz at any stage of brightness to avoid noticable flicker).
