As a disclaimer, it must be kept in mind I have not watched a lot of cricket. Most of my viewing comes from around 1990s to current day.
With that said, of all the traditional offspinners I've seen, Swann is only behind Saqlain Mushtaq.
By traditional offspin, I mean non-Murali (he was a freak), and not the fast/flat offspin style, or guys like Narine/Mendis. I mean the kind of offspinner who uses tools like flight, changes in pace and spin, drift and dip to beat batsmen. Guys like Vettori, Hauritz, Ojha, Lyon, etc. Saqlain was a beautiful bowler of that style early in his career, really amazing. Wonderful to watch. Slightly dodgy action, yes, and a bit unusual, but still a very good bowler of that style. And he had the doosra too, which made him such an incredible bowler. One of the bowlers who inspired me to bowl spin, along with Warne.
After him, I would rate Swann as the best exponent I have viewed of these classical skills. He has almost all the tools except the doosra, and while he does lack a bit of the 'magic' or the kind of zip that Mushtaq used to have, he is pretty close to it, and when he's bowling well he is almost the epitome of beautiful, brilliant offspin.
Harbhajan early in his career was too, tbh, but my fondness for him has faded a lot given how bad he has been as a Test bowler since Kumble retired. He was so good earlier, especially in that brief period where he had the doosra and was still flighting and spinning the ball. Had a lot of comparison with Mushtaq.
Having said that...I can see where Benchmark is coming from when he says Swann really isn't as amazing as we make him out to be...but I don't put that down to him not being as skilful as we say he is. My theory (or belief) is that classical offspin, even when bowled to the highest degree it can be bowled, will still be less of a wicket-taking threat than legspin, pace or swing bowling at their finest. Swann isn't limited by his skills, per say, but by the style he bowls. If he had a doosra/carrom ball/something similar, he would be more dangerous.
Having said all that, I haven't seen the likes of Laker, Gibbs, Verity, Mallett or Erappali Prassana bowl. I have only seen fleeting clips of Bedi. So I don't have much to compare against.
Ajmal doesn't fit in this category of 'classical offspin' because he does bowl a bit faster and flatter, and still gets sharp turns, and has a wicked doosra. His bowling style is less about flighting it up outside off, making the batsman drive against the turn, and trying to get through the gap between bat an pad, but more about bowling it quick and sharp-spinning in line with the stumps/just outside off, and get the batsman guessing about which way it will turn or how much it will turn. It's still wonderful to watch, but different.
Watching Ajmal and Swann, I can't help feel Ajmal is more of a threat. He seems more like a Murali...able to beat a batsmen off the pitch instead of in the air. Maybe because no batsman has worked out a way to play him safely yet, he's been effective. For Swann batsmen already know they need to use their feet decisively, and if they do he can be neutered, but he's such a smart and skilful bowler that he can work around that and still deceive and trick the batsman. Swann is such a clever spinner. I don't think Ajmal has had to deal with such a challenge yet, and if faced with a flat track that he can't get a lot of spin off, he still works out ways to beat the batsmen off the pitch. Mixing the ones that turn slightly with the one that doesn't spin at all, or holds it's line. Varying his pace a bit. Swann in the same scenario would go for beating the batsman in the air, use drift, dip and flight to do so.
Different bowlers they are, but Ajmal has seemed more effective to me by quite a distance. And it's perhaps just because of they style he bowls, rather than him being a far more skilful or intelligent bowler than Swann.
But that's just my 2 cents. Haven't seen them both excessively either tbh, so i could just be completely wrong.