A: Depends on what you want from your Jeep.
The CJ5 is a fantastic vehicle, and it’s ridiculously inexpensive. The best years for that, again, depend on what you want. The 72-75s have a Dana 44 rear and a V8 option. The 76 and newer have a more robust chassis, but a smaller engine bay. The 77-79s had a boxed frame and disc brakes. Finding 76 and newer parts is still fairly easy to do. Earlier ones … well, you’ll do a lot of searching.
The CJ6 is a strange little Jeep. I wouldn’t get one unless you’re a collector. It’s, basically, a long wheelbase CJ5.
The CJ7 was a really robust replacement for the CJ5. I liked the longer doors on mine, and the hardtop & hard doors were useable. Getting a post 1982 one won’t get you the heavy-duty parts that came on the earlier rigs, but you’ll get wider axles, Dana 300 transfer case, and it’s not that difficult to find one that came with a Dana 44 rear end. I’d recommend the straight 6 over the V8; same torque, better MPG (although MPG isn’t the reason you buy a Jeep).