I couldn't pick just one year or model but there are some I consider better than others. Using broad brush strokes, I'll start with '74 or newer models because of the 447 motor. It's easier and cheaper to get parts for. Narrowing that down, the next step would be '77 and newer because of the bigger forks and improved, linked carbs. While I do prefer the Standard models, now that I own a Special as well, it's starting to grow on me, lol. Applying the usual upgrades we do, they can be made into a very competent machine, nice riding and nice looking as well. I don't care for the rear disc and prefer a drum, not so much for performance but rather for simplicity. It's stone ax simple and there's no fluid to leak. That being said, I'd have to rate the '79 SPII as my favorite Special. It has the rear drum and it was the only Special that got alloy spoked rims.
Now let's talk about the '80 and newer models a little. These were all the Special style and only the '80 Special had a disc rear wheel (which puts it at the bottom of my Special list). Best upgrade to these later models is the factory electronic ignition. The BS34 carbs are rather pollution strangled but can be "fixed" and made to work very well. The seat grew a big, ugly hump on '81, but it is very comfortable. But the '81 also got a rear mag drum and I like that. I don't care so much for the mega-spoke Heritage wheels that came along in '82, or all the extra "safety" features they added, but you can change all this stuff. My Special is an '83 and had all that stuff. I changed it all, lol. I removed those safety relays and switches, swapped on a flatter '80 seat, and ditched the mega-spoke front wheel (I can live with the 48 spoke rear). I'm quite happy with the bike now.
As far as the '79 models go, the only one I don't care for is the regular Special with the rear mag disc. As I mentioned earlier, the SPII is one of my favorite Specials. The '79 models have the last of the BS38 carbs (and the best in my opinion) and the better 35mm forks. They still have points but you can upgrade that. I'd love to find a '79 Standard. I've never even seen one in person around my area, lol.