Usually I try to go for a thicker toy. But a lot of the thick dildos here are made from some bad material. So I'll look for the largest silicone one I can find. I mainly just try to avoid rubber, TPE and jelly dildos!
The type of dildo I'm interested in is something that isn't draggy and feels realistic and is slim enough to insert. Again, something that feels realistic. My first dildo is silicone and it felt exactly like a rubber/silicone stick ripping my insides, it doesn't feel good. I only have 1 dildo and it's so ungodly draggy even with lube, I can't even insert it and when I can insert it a little, it just feels uncomfortable and not pleasurable. So if I was looking for a dildo, I want something that isn't draggy, feels realistic and is actually pleasurable. Blah blah blah.
Quality, size, realism. I like high-quality dildos which are my "just right" size, as real in feel and appearance as possible. Vixskin is pretty close to perfect, except for practicing oral; UR3 or cyberskin is better IMO for that.
I like smooth silicone, though I do have one with some texture and a glass toy with TONS of texture. I also like a decent amount of girth and length. I prefer non-realistic dildos, since realistic ones look a bit silly to me. Color isn't especially important, though I do wish more were available besides pink, purple and skin color.
Material comes first as I prefer either glass or silicone then I take note of texture and size. Girth is more important than length and I enjoy dildos that are not smooth but have some texture to them.
I'm rather new to vaginal penetration so I've decided to stick with silicone since it's a safe, soft material. That lets me focus on size (a major concern for me) and whether I intend to use it for vaginal dilation or G-spotting. The size limit keeps me grounded, the intended purpose determines the shape and texture, if any.