From what I see, Oxford has more of a 'relaxed' vibe but we are talking about Oxford University here.
There isn't a specific college that's easiest to get into(I know this isn't the answer you want) but all colleges want different things. Theres no use looking at how many people they take in from each subject and their acceptance rates etc, you may as well apply for the college you want to go to, if another college wants you also, they will ask to have an interview with you.
It also really depends on the subject you would be applying for!