Wow. Interesting question.
I agree with Jwintjes that the old Heller Nina, Pinta, and Santa Maria make good starter kits. The problem is that, at least in the U.S., they're hard to find - not the sort of thing the beginner is likely to see in the local hobby or craft shop.
Actually an up-to-date yacht America probably would make a nice subject. Lots of people - including non-modelers - have heard of her, she has minimal rigging, and few more handsome ships have ever been built.
We need a good styrene Viking ship - a scale model of either the Oseberg or Gokstad Ship. The old Revell Gokstad Ship from 1976 was excellent; I'd be delighted if Revell reissued it. I believe one of the European companies has announced a Viking ship of some sort for the near future; I'll be interested to see it.
Some of the later America's Cup contenders - the J Boats of the 1930s, like the Rainbow or the Endeavour - would also be good subjects. One mast apiece. A couple of wood kit manufacturers have tried resin-hull J Boat kits, though, and I have the impression that they haven't sold well.
Lots of tourists in the neighborhood of the Great Lakes have seen the reconstructed War of 1812 brig Niagra. There's already a good plank-on-bulkhead kit from Model Shipways, but a plastic one would be nice. On the other hand, she actually has quite a lot of rigging - not the best beginner's subject.
More than a million people are expected to visit the Jamestown Settlement historic site during 2007. How about a set of Jamestown ships - the Discovery, Godspeed, and Susan Constant? Building them in that order would be an excellent, progressive course in ship modeling.
Many, many years ago Pyro made a plastic kit of a Chesapeake Bay skipjack - the last working sailing craft in the U.S. Skipjacks make beautiful, interesting models. The Model Shipways Willie L. Bennett is a fine kit, but it takes a lot of work to turn it into a real showpiece. A skipjack would make a nice subject for an up-to-date plastic kit. It could be fitted out with oyster dredging gear and people - and would make a fine exercise for weathering techniques. (Any active, oystering skipjack is among the dirtiest vessels ever seen.)
The archives of the old U.S. Revenue Cutter Service are full of plans of handsome ships. Maybe they aren't as well-known to the public as some, but anybody who sees a finished model of the Joe Lane or the Morris is almost guaranteed to think it's pretty. Another subject that the public seems to like is the Model Shipways Sultana. Wouldn't the Sultana (or the Halifax, or one of the other colonial schooners) make a handsome styrene kit?
What I would like to see (and I doubt that I ever will) is a big variety of accurate, well-designed sailing ship kits in styrene, offering the potential purchaser a choice of historical periods, subjects, sizes, and degrees of difficulty. The experienced modeler needs some new kits too. How about a 1/96 American clipper ship? Or a decent fishing schooner? Or the Mary Rose? (Ok, we don't know enough about the missing parts of her to reconstruct her with much confidence, but what would be wrong with an intelligent piece of guesswork - like the Revell Golden Hind?) How about some American sailing warship other than the Constitution? (The frigates from the Revolution would make fine subjects.) Or some British sailing warship other than the Victory? (Would folks who've built the Revell 1/96 Constitution be prepared to shell out the additional cash for a 1/96 Guerriere? I have no idea.)
I suspect anybody who's actually involved in the marketing of plastic kits has concluded by now that I'm hopelessly naive - and I probably am. On the other hand, the state of the plastic sailing ship kit business seems to suggest that the people in charge of it haven't done such a great job of judging the market themselves. Maybe the concept of the plastic sailing ship model just isn't economically viable. But I hope the manufacturers don't give up on the idea completely. As I understand it, Heller was actually surprised at how many people bought its Soleil Royal (a kit that I can't recommend personally, but that seems to have struck a responsive chord among the buying public). It seems there are people out there who want to buy such kits.
The bottom line, in my mind, is that the beginner ought to be encouraged to start with a well-designed kit representing an attractive, historically significant, relatively small kit on a relatively large scale. Not a huge, expensive kit that takes months or years to build, and forces the purchaser to choose between building it out of the box and getting an unimpressive result or spending lots of extra time and money to "accurize" it. Give the beginner a nice, well-detailed America, Discovery, or Sultana - a kit that he/she can turn into a nicely detailed, accurate model in a few weeks of evenings, without investing hundreds of dollars in it. That, to my notion, is the way to learn ship modeling, have fun with it - and expand the hobby.