My vote goes for the T-72/T-90 family. Quantity has a quality all of its own and while individual tanks may perform worse than the "better" NATO models or "worse" than advertised, in the end it won't matter if the commander can achieve a 3-to-1 advantage for a similar budget.
You might scoff at this, but remember there's a reason why Russian miltech is so rough: it's built for war. As far as Russians are concerned things get blown up in war, so any cent kopeck spent on polish is wasted... and if world war 3 ever rolled around, I have a feeling the Ruskies would've had a point.
When on "even terms" (i.e. your *overall* tech is not hopelessly antique and you have sufficient number of units) Russian tech tends to do a lot better then what most people give it credit for. Actually most "real wars" of the late 20th century (with maneuvers and clash of armies, instead a curb stomp battles/police action of the 500 kg gorlla called USA) were fought with Russian tech on both sides. On these battlefields the Russian tech has more than proved its doctrine in my opinion. Often operated by badly trained crews, with spurious maintenance, the tech all in all did what it was designed to do so. Let's add atrocious supply & maintenance levels to the mix and the Russian tech has another area where it's well ahead NATO tech.
Which doctrine is "better", very much depends on what the security needs of your country are - and how much money you have to spare. So in the end there's no such thing as "best" MBT (or best weapon system/platform, period).
For most of the folks actually fighting protracted wars, the choice will inevitably be Russian most of the time.
Ironically, since the US/NATO has been engaged in what are actually COIN operations for the last 10 years, the very same qualities - cheapness, ease of maintenance, low training requirements - have come to be in demand...
...but the US could never do COIN well. All it ever learns is that it can't do COIN.
(The only moral learnt in Vietnam, was not to fight in Vietnam... and seems like similar conclusions will be drawn after Iraq too).