Judging them on today's look, what Indy girl has held up the best? I vote is for Kate Capeshaw, she still is amazing to me. Alison Doody, still hot but more of a 'model' look than a 'real' girl look that Karen and Kate have.
Karen was always the true beauty, and has held up the best in my opinion. Alison looks like she paid for her face, and I have felt for many years that Kate has fallen apart. But then, I never found her very attractive to start with, so take that for what it's worth.
Can't count Cate B yet. She's hardly had a chance to age. Ask me in ten years.
The bottom line is... if you are a very good looking young women, you are more than likely (assuming you take care of yourself), going to be a very good looking middle aged/older woman. All three may be substantially older than me (well Karen and Kate anyway), but if I didn't know who they were, I still think I'd ogle them if they were stood next to me in a bar (if only I were that lucy).
Watching the Making Of DVD that comes with the trilogy Karen doesn't look all that great. But seeing her in KotCS and in the flesh last year at a screening of Raiders she looked a lot better. I don't know what she did there.
Alison has always been 'my' Indy girl, though. I think she always did and still looks the best.