I wasn't a tanker during my Army career, but rather a Redleg, and spent several years in M109A3s both as a gunbunny and section chief...
Of all the pieces I crewed, the 109 was the best one to live in. It had a turret, which meant not standing around in the rain, and a heater...
For modeling, it's a toss-up... The M4 family and Panzer IVs, I reckon... They're certainly what I've built the most of...