Well my choice is the US M1A2 Abrams Tank obviously, I'll go over the main reasons why here.
Speed
The US Abrams tank has a max speed (with regulator) of 42mph, it goes up to the mid-50s, possibly even as high as low 60s (if you push it) without it, compare the British equivalent with a 36mph top speed on the road and you see a clear advantage in several situations, cross-country, large-scale manueverability (Iraq), quick-reaction forces on a smaller scale and the ability to keep up with convoys of regular civilian/non-60-ton-army vehicles without them feeling like they're being forced to inch along.
The disadvantage of that is that the Abrams is a gas guzzler, starting with Iraq I however the US Army began to get a resupply system going, especially for cross-country it was particularly effective, allowing the Abrams to go long distances and fight with little problems. The advantage of being able to go those kinds of speeds, both in the macro (Iraq I famous 'left hook' etc) and micro (quick-reaction forces can be scrambled much...uhm...quicker) combined with the effective resupply doctrines clearly puts the Abrams at an advantage.
Ammo Compartment
Perhaps more of a psychological thing than anything else, the Abrams has a seperate ammo compartment which, if compromised, ensures that the crew survives (losing a well-trained crew and tank being considered a bad thing) if the ammo compartment is hit. There is also the whole "not surrounded by ammunition" thing, a psychological benefit if there ever was one.
The disadvantage to this (suppsoedly) is a decreased rate of fire. Compare, however: BBC lists the Challenger II's ROF as 8 targets engaged per minute, whereas the US Abrams has a ROF of 6 to 8, depending on the skill of the crew etc. Thus the problem of a lower RoF seems to be solved by some additional training. The actual prevailence of the additional training in US <strike>college bums</strike> tank crews is, however, open to debate.
Operational Readiness
As of Gulf War 1, the M1A1 Abrams tank achieved a 90% rate of operational readiness (being able to go out on a mission), Gulf War 2 has given rise to some complaints, however I have been unable to locate if this has decreased the combat readiness or if these have been (somewhat) isolated incidents, just seemingly more prevailent due to the tanks being out there more often. So I am sticking with the 90% rate, that being the closest operation data and hopefully teh US army didn't just add stuff in figuring the tank would never encounter a grain of sand ever again.
Still, 90%, especially in the desert, is amazingly high and worthy of note.