I'm coming late to this thread but I have had a look around past posts.
My vote went for the Challenger 2. Mostly as it can use HESH. There is no limit to the type of target HESH will take out and range is no bar. A HESH round arriving at extreme range is just as effective as one from 200m (if not as easy to get onto the target). HEAT is too easily defeated by modern armour and KE weapons loose energy with range.
The versatility of 12cm HESH rounds is vital for the proper use of a tank. Tanks don't exist to kill other tanks. They have to be able to that so that they can carry on their proper task which is to support infantry.
I vaguely recall a comment, from someone in a position to know, that reactive armour is a handy addition as it adds to the bang when a HESH round arrives.