My personal favorite is the Mark VI. I know this has been stated a million times here, but what the hell, I might as well through out another good review.
I play on a alto mark Vi from the late sixties. If you want I can post the serial number but it`s upstairs right now. sry. It is the best horn I have ever played on.

I played a yamaha 875, black laquered, a pure beauty next to the old worn mark VI, and while it had excellent tone, and was incredibly freeblowing (the low Bb felt like a G) It was a great horn. but... the Mark VI, while not as freeblowing (and no high F#) just has this bite to the tone, this edge, the most beautiful sounding horn I have ever laid hands on. It`s amazing.

The reason I think they can`t match horns today eith the horns from back then is how they work the metal. Before saxes were machined, they were made by hand or whatever, but instead of the metal (brass bronze whatever) being banged on only once, in the old days, they would re-heat them three or more times and bang on them or whatever agian. I do some blacksmithing, and each time you heat and re-beat the metal it gets harder. I believe this is the main difference from old and new saxes.

Sorry about my lack of technical terms above, this is a vague theory and I havn`t researched it enough to know the terms or what the did and so on.

From what I have seen, both from playing saxes, and working in music stores, I have learned one true fact. An instrument is like a snowflake; no two are the same. Yes, today`s instruments are mass produced, so the differences are miniscule, however the differences are still there. What the decision should boil down to is what horn fits for you. Yes, the Mark VI gives more of a bite to the sound and some players like that feel. On the downside, most Mark VI`s will not stay in tune as well as the newer models. Contrary to what one person said in an earlier response to this question, the Series III tends to play brighter, which is due to a few features. One is the metal, mainly because it is not reheated as much as in the olden days. Two, is because of the metal resonators being used rather than plastic resonators. This horn will stay in tune better than most older horns, however no horn, especially when it comes to a saxophone, is perfect. I personally like the original super action 80 (tenor) and the Series II (alto). I have found these horns to give a quality sound for whatever style I may being playing for that day. For those that are reading these responses when deciding what horn to purchase, remember, it`s not just the horn that makes the sound. You must also include the mouthpiece and the reeds. For a classical sound, I play on a C* and Vandoren traditional reeds. For jazz, I have used Meyer, Link, and Ponzol mouthpieces with V16, Java, and Rico Jazz Select Reeds. Just remember, when reading these responses, take into consideration that these are my personal preferences. As for my feelings on the horns that have been discussed, those are personal opinions too. The best thing that anyone can do is try as much as possible; whether it be saxes, mouthpieces or reeds. The quest should never end.

just what I would say... it`s personal choice. Me? I love selmer mark VI`s and wouldn`t think of anything else.
on the other hand, the other saxes are great to, for exampe, I played a yamaha 875 alto, and it was soo freeblowing i loved it, BUT, it lacked the bite that the mark vi has