If you want a pure touch-screen phone then get one with a capacitive screen. The iPhone is the best that I've tried, though I've heard good things about the Samsung i8910, and the HTC Hero is also good (although the screen is quite a bit smaller, which doesn't help).
You might, however, consider getting a phone that has a physical keyboard as well as a touch-screen. I have an HTC Touch Pro 2 and am loving it so far. It's not for everybody - it has some great business-oriented features, but the camera isn't too hot, and it's thicker and heavier than most phones; but the keyboard is just wonderful.
There are plenty of other phones with keyboards, of course. I'd avoid the Nokia N97 if I were you, but something in the Blackberry range, or a Nokia E71 or E72 might work. Or a Sony Ericsson Xperia X1. Personally I like the Touch Pro 2 better than most of these, not least because the keys on the keyboard are better spaced; but if you girlfriend has small, dexterous fingers she'll be okay.