The Leopard 2 and the M1 are very close to being the same vehicle, they're both products of the MBT-70 program and they both use more or less the same gun. Both they and the Challenger 2 use the same armor.
The M1 has the best electronics, and it also comes with the widest variety of extra refit options. The Tank Urban Survival Kit in particular is worthy of note and a lot of that is going to go into the M1A3 when it's finalized.
The Leopard 2 has pretty much everything the M1 does, but the Germans don't have the funding the US does so everything isn't quite as nice or up-to-date. They did probably keep the best 120mm guns for themselves, though.
The Challenger 2 is unable to fire NATO standard rounds because its 120mm gun is rifled. This gives it a longer range but denies it access to the kinetic-kill penetrators the Abrams and Leopard use. Given the outbreak of ceramic or reactive armor a rack of HEAT ammo isn't as threatening as it used to be. (There have also been concerns about the reliability of the ammunition in the UK's stores in recent times.)
The T-90A is essentially an unknown factor, but it comes from a design lineage that has not stood up well in practice. The T-72 has been consistently defeated by Western vehicles. (I also notice whoever designed the turret hadn't heard of shot traps.) The multi-layered protection system of composite and reactive armor is interesting but hasn't ever seen a real test. It's essentially a stopgag vehicle until the T-99 comes into service in any case.
The Merkava IV's only combat outings have not been of great success, though this reflects the operators rather than the vehicle. The ability to carry a squad of infantry is much-touted, but it comes at the cost of not carrying ammunition, which means it would be of minimal use in most situations. The Israeli composite armor isn't quite as good as that on the other western vehicles, and at least one Merkava IV was penetrated by a missile that didn't actually explode. There have been noises since 2006 about terminating the line and designing a new tank, but the Israelis have staked a lot on their active protection systems to keep the Merkava competitive.
I would personally prefer Leopard 2 or M1 over the others, but in a sense I'd rather defer until 2015 and see what the M1A3 and its contemporaries look like.