Normally, I wouldn't even attempt to answer a question like this because so much depends on all the considerations that Piers mentioned and many more. However, if I had to stick my neck out in terms of what would appeal to seasoned collectors and newbies alike, I would have to say that a proper akoda nari kabuto from the Muromachi period with it's unique-to-Japan, so-called "pumpkin" shape would have to be up there (even more so if complete with a haramaki or domaru, but that's probably more for connoisseurs as they are so rare and require some knowledge and scholarship to ensure a proper period piece and not an Edo revival). Also, a kawari kabuto from the Momoyama period, with their flamboyant, sculptural shapes. As mentioned, the Saotome are amongst the best kabuto makers of the Edo period. These are items that even the layman would appreciate. For connoisseurs, we get into some of the more esoteric items and schools: the Yashu; Joshu; Sansaku; Bamen; Saika; Nagasone; Unkai; etc.. And then you can drill down within categories. Even the common zunari offers a range of possibilities: ko-zunari; ballistic zunari; hineno zunari; etchu zunari; etc.. Very, very loaded question, with infinite answers really.