Honestly, I would do a little more research and figure out what you want to achieve.
"Best emitter" is far too vague, as it really depends on what you want from the light. And popular emitters don't always mean best.
I would also seriously consider changing the driver too. Unless you know the driver you have will work well.
And maybe consider battery options too.
First up, you need to know what size LED star you need and what size driver. This will largely dictate on what will easily fit.
If you want highish output, it's well worth making sure you buy a LED mounted on a DTP (direct thermal transfer) star, usually made of copper. There are several about, be it Sinkpads or Noctigons or someone else. You can buy them pre-mounted with an LED and in a choice of sizes.
There is a nice choice to be found here:
http://intl-outdoor.com/led-c-107.html
With regards to which emitter, you need to think about what sort of tint you'd like, such as a warm white, neutral or cool white. The CRI rating and the beam type, i.e. do you want a large hot spot, bright spill and more floody. Or do you want a smaller tighter hot spot, better throw, but duller spill beam.
CREE's current offerings are:
XP-E2 - this is a very small LED. Output is lower and generally not found in as high CRI ratings and usually only cooler tints. Should be efficient at lower amp draws and will offer good throw with a small tight hot spot.
XP-G2 - slightly larger than the XP-E2 with higher output and a larger hot spot, but still quite good for throw. Nice choice of tints and CRI's.
XM-L2 - Large LED with a lower surface birghtness, this will be a much larger hot spot and brighter spill beam, but a much more floody light. It is more of a powerhouse and offers the highest lumens however, but remember lumens are a bit like torque with a car engine. How it's used doesn't always mean the biggest number is the best for your application. Being a bigger LED you'll need a larger hole in the reflector (if it isn't already).
XP-L - This is essentially an XM-L2 dye on an XP-G2 pad (pad isn't the same as star). This means it has almost identical performance and beam to the XM-L2, but the dome is XP-G2 sized, so will fit in reflectors with smaller holes.
MT-G2 - very large LED and high output, probably not ideal for your build unless you specifically design it to work with it.
The Nichia219B is also worth considering. Similar to an XP-G2 with slightly less output, but higher rated CRI. Although it's a close call with some of the NW XP-G2's.
As for drivers, I'm willing to bet your current one doesn't really supply the amps.
An XP-E2 will run on 2.2 - 2.4 amps for max output. Although 1.4 amps is a nice entry point.
The XP-G2, XM-L2 & XP-L all deserve 2.6-3.0amps to get the most from them.
The Nichia is happy at around 2.4 amps, although they will run on a 3.0 amp driver quite well, just get hotter quicker.
