Whichever one(s) you like best. It's not like they are going to be worth tons of money some day. I prefer the 1x4/5 series but I really like them all. Here's the order I like them, as far as series go:
Individual models order of preference:
125 (3), 105 (2), Original (1), 1450 (1)
I have these as parts tractors: 104 (2), 107 (1), 149 (1), 125 (2)
I think the Originals are the most collectible, especially with all of the options intact. That being said, if you're going to use it, the Originals aren't quite as robust as the direct-drive models, starting with the 70 and 100. The Original has a belt drive, plus the frame bolts on to the front of the transaxle, and have been known to crack.
My favorite is the Original, followed by the 70 and 100. There are hard-to-find parts that run the value up, such as the oil-filled air cleaner, fenders, creeper drive, lights, etc.
Ultimately, it comes down to what you like, as I don't think anyone is going to get rich dealing in Cub Cadets unless you have a warehouse full of NOS tractors hidden somewhere. I use my CC's, although plans are to do a full restoration or two.