In general every woman we meet here is top tier so it's not easy to make a choice when we have to.
For me, the "nice" thing about AVNs is that looks just aren't much of a factor in choosing a favorite LI--they're pretty much all at least 8/10, so even if the one I like best isn't the one I think is best-looking, she's still plenty good-looking. Instead, my ranking has more to do with the relationship with MC. My favorite of the 1999 LIs has to be Jenny--she's totally, helplessly, head-over-heels in love with MC, and I'm 100% here for it (I'm a strong advocate for MC to just F the rest of the story, stay in 1999, and marry Jenny). Second is likely Luna--she wasn't there before Cindy showed up, but she was well on her way. For looks in 1999, maybe Misty, maybe Luna--Scarlett's undeniably hot, but I tend to prefer a slimmer build.
For 2019, probably Skylar. She'd probably top my "looks" list too, if not for the ink, but that is a negative in my view (even as I recognize it as important to the plot). Alyssa is pretty high on both lists. Shelby's pretty high on my "looks" list, even if I can't stand her.
Darci's kind of an interesting case. The childhood friend is usually a shoe-in for me (I adore Annie in Eternum), but Darci isn't doing it for me so far. I think it goes back to the relationship--maybe there's some latent romantic interest on her part (or MC's), but neither is admitting it so far, and without that she isn't that interesting to me.
And I don't know WTF is going on with Riley. Her comments in Cindy's bedroom suggest quite a bit stronger devotion to MC personally than we've seen shown to this point. But I think her relationship status in the phone is still shown as ????, and that's where I'm at too.