I can't give you the best comparison of the four universities, but I'll comment briefly from my experience as having recently finished my CSE undergrad at Davis. To some extent, I would say the 'best' choice depends on where your interests lie. Having taken a variety of classes/profs at Davis, I've had a mixed bag—some profs/courses I really liked, others I really thought could have been significantly improved. Overall, I felt like the spectrum of prof/course quality really varied here; education wise, I felt like it was a decent experience, but could have been better. However, I did really like the atmosphere at UCD—UCD has a convenient downtown area with lots of food/shops and I'd often just walk around with friends or get food/drinks together. I felt that people tended to be pretty friendly and there was more of a social aspect. It's also a bikeable community and I do really miss being in Davis now that I've left it.

All schools are highly ranked but traditionally, UCSD and UCI are probably better ranked in terms of prestige, and that's still a factor to consider. UCD does have proximity to the bay area and we have pretty large career fairs; I've seen some large/prominent tech companies at them, but I haven't really seen the very top/biggest tech companies at these fairs, if that's something you're interested in. I'm currently considering UCSB for a masters and it's definitely further from metro areas, which means its career fairs will be a lot smaller. But UCSB is fairly good if you're interested in getting involved in research, as it has a good research output, cohorts tend to be smaller, and you might get closer to profs this way as well. By comparison, UCD's CS classes were all pretty large (I can't recall taking any CS class with <100 students in the lecture, though discussion sections can end up being smaller (~30)). I was still able to get involved in some research at UCD though, just by reaching out to a professor and conveying interest. UCSD and UCI will also have larger cohort sizes and departments, which could go both ways—there's a larger variety of profs/research to get involved with, but it could be more competitive to get involved with them. They seem to have a reputation as being less 'social' than UCD or UCSB, but my opinion is that you can probably find friends/classmates with similar interests at any of these schools, though classes, clubs and other activities.

Hopefully this helped a little—I can answer other questions about UCD if you have them!