Well... It's not very imaginative of me, but I think jewelry and/or flowers will always be the most romantic gifts. Adding a romantic dinner for two is nice too. It doesn't have to be an expensive restaurant - just a romantic one. (The jewelry and/or flowers don't have to be expensive either, at least to a lot of women - just nice and something in keeping with her tastes in color, style, etc.) In fact, that may be the key: Giving the kind of flowers that aren't a type or color she prefers, or giving a piece of jewelry that's no the style or kind she ever wears, isn't romantic. Those things are only romantic if they're the kind she really likes. If they turn out to be something that reminds her of how little you really know her tastes, she may still think, "It's really just the thought that counts," but it won't be the same as if the gift is exactly the kind of thing she loves.
Personal opinion: Stay away from gifts that you think she OUGHT to like "once she's got it and sees the light about how great it is", and gifts that you think she'll look good in, or that you'd like to see her wearing. Valentine's Day is not the time to highlight how little you know her, understand her, respect her personal tastes and/or would like to have a say in what she wears. Also, don't make the mistake of thinking that how expensive it is determines how much she'll like/love it or how romantic it is.