Third generation NATO tanks, mid-to-late eighties and early nineties. Useful for Cold War what-ifs and 1991 Gulf War.
United States of America - M1 and M1A1 Abrams
United Kingdom - Challenger 1 (Mk.2 ?)
Having the above would complement the US and UK NATO models already available, and provide 3rd Gen. tanks to go up against the Soviet 2nd Gen. T-62, T-64, (and to be released T-72) tanks. Someone will probably ask for the Soviet T-80 3rd Gen. tank, I'd imagine.