Personally, I would have to say the older American LRRP (Long Range Reconnaissance Patrol) units, though I say that both due to the fact that my father was one back when he was in the army, and the fact that I know very little about the other units in the poll. However, I DO know that LRRPs were essentially the other side of the same proverbial coin that the Rangers belong to. Rangers have sheer numbers, are loud, proud, and are the U.S. Armys 'elite' shock troops. LRRPs on the other hand, are small units that are quiet, rely on stealth, as well as operate behind enemy lines. The ratio of enemy kills to LRRP deaths is also extraordinary, sometimes going up to 400 enemy casualties for every death of their own (As stated in their wikipedia entry: http://en.wikipedia.org/wiki/LRRP ).
However, like I said, that's mainly due to favoritism. Other than the LRRPs, I would definitely have to say the Gurkha's of the British Armed Forces. By reputation, ice not only runs through their veins, but they are also the nicest people you could have on your side. Personally, if I was ever in a situation against a Gurkha, especially in wartime, I would piss myself. (For a more in-depth look into the Gurkha's, here is their wiki entry: http://en.wikipedia.org/wiki/Ghurka )
However, like I've said before, I know next to nothing about the other Special Ops groups listed in the poll, so my opinion probably doesn't, or shouldn't mean much. I would never discount any Special Ops group in any National Armed Forces, or even one that belongs in a Private Military Company.