In terms of percentage of electrical input converted to visible light the best I've heard of so far are the EZBright blue dice made by Cree. The best bins produce 30 to 33 mW of 460 nm blue light from a typical power input of 64 mW, making them 47% to 52% efficient. Note however that since the eye is relatively insensitive to blue light these only have a visual efficiency of about 30 lumens per watt. The best production white LEDs produce around 80 to 85 lm/W but in terms of conversion efficiency this is only around 25%. This is true of both the small, low power 5mm ones and the higher powered ones like the Cree XR-E. Nichia is supposed to start selling a 5mm white LED of 100 lm/W efficiency around now but I haven't heard of it so far. Next year both Cree and Seoul Semiconductor will be selling 100 lm/W white power LEDs. I have heard of some infrared LEDs which convert about 60% of the input power to light. Of course, the light produced by these can't be seen by human eyes.
In terms of raw conversion efficiency the approximate best numbers I've seen so far by color are as follows:
blue: 52%
true green: 14%
red-orange: 25%
red: 40%
yellow green: 5% (?)
infrared: 60%
white (blue plus YAG phosphor): 40% (this is the 131 lm/W sample made by Cree)
I've heard of infrared semiconductor lasers which have gotten around 70% efficiency at room temperature and as high as 85% when cooled to cryogenic temperatures. While these numbers are indicative of what may happen with visible LEDs I suspect it will be some time, if ever, before LEDs reach 90% efficiency. It is theoretically possible, though.
