Alright, here's what I've got to offer you.
I've had a Systema M150 gearbox in one of my primary's for probably 5 years now (with a Guarder SP120) spring in it.
It comes with helical super torque of gears, which are awesome. Even after 5 years, they STILL show no signs of wear. However, the gearbox also have faults. The first fault is the Systema piston. They are utter garbage. Replace is right away, as I've had one strip out in less that 100 shots. It's a full tooth piston, you'll want to replace it with a half tooth piston - I recommend a Madbull piston. I've had nothing but good luck with them, and it's what I've had in mine since it stripped out. The other problem is the mosfets drain your battery if it's plugged in, even without you shooting. Just be aware not to leave a battery plugged in during prolonged times of non-use (a day of airsoft is going to be a problem though). Next, Systema springs suck. They're inconsistent and are prone to breaking. Throw a Guarder SP140-150 in there and you'll be happy. Lastly, I don't have any pictures, but they are known to have a weak 'peg' in the gearbox, which holds the electrical contact in place. I've had one break, but not from this particular gearbox.
Next, I've also had an M170 mechbox, which I again put a smaller spring in. Not so great. They come with different pistons - theyre Area 1000, which are made by Systema, but are equally as crappy. They also don't come with a bearing spring guide, which is definitely the optimal choice for high velocity setups. Other than that, same problems as with the M150 boxes.
The Revolution I've had no personal experience with. I hear they're great - but not in a high velocity setup that a DMR dictates. They're made more for the M110 range, and anything beyond that you're prone to problems.
The Energy Series, I've also had no experience with and really haven't hear much about.
(As a sidenote, if you're sticking to semi-auto only, get an 11.1v 20C lipo. Not over kill, quicker trigger response, and overall better than a 7.4v or any NiCd or NiMH on the market.)
If you're looking to piece together a gearbox, which is fun and you learn a lot - though I advise against it unless you have fairly extensive experience with an AEG. Most of my gearboxes have been pieced together by myself, but it really can be a very frustrating, especially with limited experience.
Anyways, if you opt for that route, Guarder are quality internals, for a very good value. However, you need to be careful what gearbox you use because they cylinder heads have abnormally small positioning holes in them. Prometheus is also extremely good quality, though on the more expensive side. Their pistons as someone else noted are extremely nice, but if you don't have perfect alignment and shimming, you're gonna strip it right away, and they're fairly expensive in terms of pistons. Prometheus springs are also junk as well. Systema is also a great choice too, just again be aware of their junk spring and pistons.
That's about all I can think of as of now, but if I think of more, I'll add to it. Or, if you have more specific questions, let me know.
Ahh, motors. Forgot about those. Guarder, G&P (I think they're M140 Motors is what they're called), Hummer, or Systema Magnum would all be great choices. All of them are high torque. With the Systema, if you don't have proper alignment, you're going to strip out the pinion gear - they're designed for that to avoid stripping the pinion gear, but be aware. Also, they Magnum can eat through brushes too. If you do get the magnum and strip out the pinion, replace it with the 'hardened' version made for it.
My personal favorite motor out there is a Systema Torque Up, but they haven't been made since the Magnum/Turbo series came out, so best of luck finding one.