Hard to beat the classic Sherman. Basic design from 1941-42, produced 1942-45 and still in combat service in 1973. Still in limited use until the 21st Century. It's actually hard to choose between "Sherman" and "Firefly" which was just an upgunned Sherman - one that could reach out and touch somebody! Everybody rember to count the "Firefly" votes with the "Sherman" votes at the end.
Modern tanks? I was always enamored of the M60 series because I grew up during the Cold War, but the M1-series Abrams is the proven "King of the Killing Zone"!
Personal favorite if I won the lottery and could actually have a tank - M24 Chaffee. I always liked this little guy, plus its light enough to use for parades, etc.
Gary B.