That's not a simple question. It's not as if someone here would even be able to give you a definitive answer, like "model XXXX LEDs from manufacturer YYYY are the highest efficiency". You really are going to have to do some research here on your own. What kind of LEDs are you looking for anyhow? is this for illumination purposes (as in LED light bulbs as opposed to little blinkiing lights on the panel of a piece of electronic equipment)? How much light (how many lumens) are you looking for? What's the power source? form factor of the LEDs?
You can see that there are a lot of variables here, and since there are about a zillion and a half LEDs available on the planet (and, as Ron points out, new ones are continually coming onto the market), you really need to narrow down your search at least somewhat before we can really help you.
Can you give us some more details?
By the way, before you become invested too far into your search for high-efficiency LEDs, you might want to read
this article ("The LED's Dark Secret" from the IEEE, not from some Internet rumor-mill) about, basically, why LEDs are not ready for prime time in terms of replacing CFLs and incandescent bulbs for lighting. Turns out that LEDs hit a brick wall efficiency-wise above a certain size and fall far short of other lighting technologies, like CFL, in efficiency. This problem, of course, is being worked on, but hasn't been overcome yet.