Unless there is a college that has a particular draw, then the advice above about one old, one new, one centre, one outlier is basically what we followed. If you've seen one example of all of the above - making sure they offer your child's chosen subject - and attended eg admission talk, meet the tutors, rooms tours plus visited the faculty then from there you can do further research online as to which colleges accommodate for all years and so forth. The colleges can be very welcoming outside of official open days if you need to do a return visit, though obviously depending on when you do that there might be a certain vibe and energy missing compared to a big open day.
We went along having done very minimal research. Nobody in our family has been to Oxbridge and the school sends less than a handful each year and has only really started doing that recently. I asked my daughter if she wanted to go as I thought that from her grade/supra curriculars/personality profile she could be a good fit and that she would be silly not to at least consider it. That said, we went along with no expectations and mostly were just planning on having a fun time together in a cool city we hadn't been to before whilst looking at how things can be done at the top end.
My daughter fell in love with everything about the place. I thought she would be strategic about her college choice on her application but she went with her heart and put down one of the grand ones where she said that it just 'felt right'.
@StrangewaysHereWeCome This is the position my daughter was in having been made an open offer from her chosen college. She did her offer holders day there too. As events turned out, they confirmed her place on results day and that is where she has just completed her first year.
I made a comment to her when I picked her up just recently about whether she's happy with her university choice and she replied 'absolutely and I definitely picked the right college'. Though, I reckon she'd be saying that whichever college she ended up at as the loyalty is strong.
Mostly, just try to enjoy it, it can be a lot of fun albeit hot and with a lot of walking involved!