Personally, i like items that have to do with plants/growth or garden, animals, crystally or magical things, then the other group is death, decay, disease, daemons, bones and skulls and all that stuff. blood too. the void. oblivion. all that good stuff.
My favorites are ones that overlap. i really liked "a virulent rose" for instance. a flower, but also diseased. "toxic shrubbery" would be the same group. a nice shrub, but dangerously poisonous.
The Atlantic Statue "A Paradoxical Statue Giving A Reverent Form To The Formless Lady Called Oblivion And The Void" is my favorite example of an item that brings the 2 groups of what i like to collect together.
Perfect example of what i would call an excellent rare. Solid name, storyline, color, graphic, and fits with my theme.
So i have to "like" something, it has to fit into one of my two broad collections. There are many things i like, really cool rares that i just wouldnt want because they dont fit into my personal deco schemes.
After i "Like" it, and it fits, then its best for it to be a "good" rare. by good i mean solid name, something that fits and isnt just random. im a fan of the "literary sounding" long names, although not every item has to have a long name to have the name be solid. A name that fits well relating to the story (if it is an EM rare)
Rarity of numbers and uniqueness of graphic and color used, also if its been duped is of course also a consideration.
Color is always a factor. and size. flashy colors are nice, but i prefer a solid name with a solid storyline, plain color, rather than a terrible rare with an overused graphic, mispelled name, but happens to be glacial or abyssal in color.
Has to have at least some type of history or "connection" to the UO world. Mesanna items are very very weakly connected to UO so are not desireable to me at all. they look cool but thats about it. No real substance. pixel crack with a devs name on it and nothing more. Same with random EM created items like the glacial lantern for instance. Just pixel crack. No real history or subtance. Not valuable to me personally.