Whoa, I don't even know where to begin answering that!
I guess, to start, I like the subject to be a person or people. I prefer human (or humanoid) subjects to scenes or objects, though there are exceptions.
I don't have much of a preference when it comes to the medium. If it's black and white and very minimalist, that's great. If it's colorful, that's great. If it's paint, great. If it's hand-drawn, great. If it's digital, great.
I tend to be attracted to presentations with shadows and depth--things that look shiny and include shadows. I like when something looks real in space, and I like when the art depicts something happening versus a static image--you can tell that the people are playing, or going somewhere, they look like this is a picture from their actual life. You can see emotion somewhere--you know how the subject feels about where they are or what they're doing.
That said, I do like when art is evocative of a person in a place. Places have detail, and detail lends authenticity to the person's environment. I'm super terrible at this in my art--I'm very minimalist when it comes to backgrounds, so I really admire it when it's GOOD. A great example: Allison Bechdel.
Even when it's cartoony, it inhabits a realistic space and feels authentic. You understand what's going on and how you're meant to feel about it.
I like art of many different types, but the best for me is when it's about someone feeling something, and I think it's good art when the pictures can reliably communicate what those feelings are.