The challenger II saw combat, I think it even set a record.
I'd put the challenger II down as tied with the JGDF type90 for fourth place.
1rst would be the strv-122
2nd would be the leo 2a6
3rd would be the leo 2a5
4th would be the m1a2
5th would be the leclerc
6th would be the type90 or challengerII
Japan's type90 is a leo 2 with an abrams gun, and some secret super donkey man target acquisition system that nobody can talk about.
The merkava mkIV is butt ugly but supposed to be uniquely suited for sand. The arjun is supposed to be good but it's very new, nobody knows much about it. China is supposedly coming out with a monster mbt with a 156mm gun. As for crews, the Gerris and Sweedes are still kicking tulips at training centers.
Sweedes have the coolest tanks, first the S tank, now the strv122(leo2a6ES)
From 10 years ago.
http://www.globalsecurity.org/military/library/report/call/call_98-12_a7ct3q98.htm