DBZ Characters :
Although most of us know almost all the characters on the show and their specific entrances and exits, this section of the characters and their rarity is interesting, and hopefully somewhat informative and to be continually updated.
Most rare characters who still have show significance come from the two TV Specials. The Z Warriors all appear for just about 10-15 seconds in the Trunks Special, and then about 5 seconds[Vegeta gets 15] in Ghosts From Tomorrow. Especially Goku, who is always dead for obvious reasons. Personally, I find Super Saiyan Mirai Vegeta interesting, but he's seen almost not at all[Saiyan's Wolf Lair actually has a Genga of him].
But also from this alternate future comes another future, the one where Cell comes to the present from. The Trunks of that timeline is seen for a few seconds, as are another Android 17, 18 & 20. These specific versions of these characters are very hard to distinguish from other versions, but it's somewhat interesting nonetheless.
Mirai Gohan is a rather rare character, but not quite as much as the above. He sees about 10-15 minutes of air time on the Trunks Special and 1 minute on Ghosts From Tomorrow. As a Super Saiyan, he is seen for about 5 minutes and 30 seconds respectively.
Mirai Trunks is not rare, however extremely popular. We all know this :)
Anyway, in the regular timeline, many characters are somewhat rare. Various outfits abound, but some variances are especially interesting. There's probably a lot of stuff from later Dragonball. From DBZ, Early Vegeta with green armor and brown hair is hard to find. Goku in the 'Yardrat' costume is rare, especially as a Super Saiyan. Cell Saga gives the various powered-up well-drawn forms of the Saiyans, Trunks Ascendant, and Gohan in Piccolo's outfit. SS2 Gohan goes without saying, and with lots of money.
Past that, Buu Saga has a lot. I'll do that later.