Best answer is it depends.
Personally I like the first Cub I ever bought the best - a 1450. The only one I might like better is the 1650 It has a hydraulic lift deck and a lift arm on the back. It also has an electric PTO clutch that has worked flawlessly for the 10 years I've owned it. If you go smaller than a 1450 the lift is manual. If you go older the PTO clutch is also manual.
If you go newer than than the Quietline the rear casting is probably aluminum. The KT17 2 cylinder models from the early 80s are nowhere near as reliable as the older 1 cylinder K321 or K341 - even if you get the later series II engines. And they cost an enormous amount of money to repair (think chevy small block overhaul prices).