I think it depends on a couple of things:
--what she wants to do with a dildo (e.g., g-spot vs. thrusting) ... what her hopes and dreams are sexually (cue violin music)
--what she likes and doesn't about material/texture, and I'd guess that lots of people don't know for sure, but they do have an idea
--what your relationship is to her
--how she feels about penises (also, if she has a partner, how they feel about these things. Nothing worse than a sex toy you feel like you can only use behind someone's back because their jealous of your dildo.
So, for example, I like my penises to come attached to human bodies, with the whole, y'know, alive thing going on there. That means that I don't particularly like penis-y looking dildos. I'd rather have something like one of the following:
1. Ella, who is stunningly beautiful but intended to excel at one thing, though she can also thrust from the other end.
2. Something glass that shows off a range of what glass can do, so like texture on one part but not another; maybe 2 different sized ends. Maybe something like the Silenzio? Or one of those double-ended ones that have different stuff going on at each end.
--something silicone that seemed just ideal for me and was aiming for a perfect shape to pleasure me but wasn't creating (additional) insecurities in my partner. Like .. God I have been sounding like a broken-record ad for these dildos, but the Tantus O2 Flurry, Cush, or revolution. Revolution is maybe big for a new user; Cush might be too, but lots of people love it. Flurry is more slender.