I voted Christopher Reeve and Henry Cavill, because I'm not sure I could say that I prefer one over the other. Both are the definitive Superman for their respective generations (George Reeves also belongs in that category, but he ranks below because I have a personal preference for the more contemporary takes).
It's hard to compare Reeve and Cavill because we have yet to see enough of Cavill playing DP Clark to make a fair comparison. As others have pointed out, he was really playing Kal-El in this movie, though we definitely saw the beginning of his "Superman" persona as well.
Thing is, each of the Superman actors brought something unique and different to their performances. If I had to rank them, the list would probably look like this, in descending order from favourite to least favourite:
1. Christopher Reeve and Henry Cavill (TIE)
Reeve played the Silver Age Superman to absolute perfection. His Superman was the Big Blue Boy Scout, an appealing throwback to '50s values in post-Vietnam/Watergate America. His Clark Kent was by far the most convincing instance yet of an actor making Clark and Superman look like two different people. Even if the bumbling routine was more likely to draw attention, it's still funny and enjoyable to watch.
Cavill for me is the most relatable Superman. You get a feeling for how being an alien with superpowers might actually affect someone in real life. Not counting Reeve's evil Superman in Superman III, we've never seen Supes get angry and let loose like we have in MOS. Cavill brought an element of aggression to the character, but always keeps any anger in check unless fighting a villain like Zod.
When we first see him he's lost, lonely soul, unsure of his place in the world (which Jeff East really did for Reeve in STM). But it's also clear that he's a nice guy. I thought the best early instance of Cavill conveying Supes' warm and caring nature was when he helps Lois on the scout ship. His eyes say it all. And when he finally dons the suit, we see him become the authoritative Superman we all know ("Don't play games with me, General.").
Also, it goes without saying that Cavill was by far the most imposing Superman ever physically.
2. George Reeves
The greatness of Reeves' performance stemmed from his affable, fatherly nature. I love how he plays the square '50s Superman to the tee, but at the same time, he conveys an air of gravitas that somehow, despite the silly costume, subtly makes you think "I don't want to mess with this guy."
I liked his Clark Kent, but at the same time I have to deduct points from the fact that he basically plays Kent the exact same way he plays Superman. I enjoy it on an ironic level how nobody can tell Clark and Superman apart in this version, but ideally I'd like a little effort to distinguish the two other than glasses.
3. Tom Welling
Welling comes next because I was a regular Smallville viewer for the longest time, I loved the show and thought it was by far the best version of Superman's youth that I'd ever seen. I kind of stopped watching after Season 7 or so because the quality had been declining for a while. But as for Clark, I thought Welling was pretty good in the role.
He's not a great actor and can be a bit stiff, but he actually did a really good job, particularly in the early seasons, of playing the confused and angsty teen with superpowers. Ultimately, they took too long setting him up his journey and you started to wonder why this guy was still hanging around Smallville. Even though Welling had no interest in playing Superman per se, I think he would have done alright in the role (certainly better than Dean Cain). Which brings us to:
4. Dean Cain
Embodied the post-Crisis Clark Kent, for better or for worse. Cain made Clark into a real person as opposed to a caricature or a front for Superman.
Unfortunately, his Superman was not very good at all. Like George Reeves, he made little effort to distinguish Clark from Superman, but while Reeves sounded authoritative, Cain seemed a bit ... I don't want to say whiny, but I distinctly remember as a kid excited to watch Lois and Clark and then becoming disenchanted because it always seemed like Clark didn't really want to be Superman and was more interested in pursuing Lois Lane.
Still, I haven't watched the show since then, and being a couple decades older I would probably appreciate it more.
5. Brandon Routh
Let's just say I wasn't blown away by his acting abilities. A lot of the fault here rests with Bryan Singer, who told Routh to mimic Christopher Reeve's performance as Superman. But I'll give Routh his due where Clark Kent is concerned; he managed to make him different from Superman in a more subtle way than Reeve. Reeve is more enjoyable to watch of course, but I can still appreciate Routh's contribution in that one area.
6. Kirk Alyn
Only seen a few clips of the '40s serials and they look pretty goofy, the kind of thing I can enjoy in an ironic way (and also in an unironic way, because hey, it's Superman). Alyn seems to do a good job at being a bit more subdued as Clark. His Superman looks a bit overly excited in the way he moves sometimes, due to Alyn's background as a dancer.