Emmanuel college for a number of reasons:
1) Nice architecture and grounds, most of which you can walk on. Being founded in 1584 means it has had time to accumulate some nice buildings. Front court is lovely, as is the rather powdery blue main hall. The old library is pretty old (you can actually see the original stonework if you open up one of the panels on the walls that isn't locked). The paddock is huge, and you can walk on it, and they convert it to tennis courts during the summer. Just a smidgen of what's actually there :P
2) Certain subjects (like medicine) you get teaching staff in college who take supervisions for you. Means you don't need to run all over the place to get supervised. NB: This bit needs patching up for other subjects, since I just realised that lots of colleges accomodate for medics in this fashion.
3)Accomodation is some of the cheapest undergrad rooms in Cambridge, with around £60 for the cheapest rooms, sheer size for between £80 - 85, and full blown ensuites (available for 3rd years only in college) for about £95. Pretty good if you ask me ^__^
4) Decent library and reading space. Pretty well stocked. They're currently rebuilding one of the areas at the back of the library for some more reading rooms and space.
5a) One of THE reasons to go - you feel that at Emmanuel college you are at the centre of a lot of shops, bars, restaurants. There's a off-license right outside college accomodation, though I must say that Sainsbury Sussex gets it better than us in that respect.
5b) Equally unlike Kings or Trinity or Johns you are out of the way of the touristy area. Thus there isn't a constant rush to get your photos taken or whatnot.
6) Bar's cheap - £1.20 for a pint last time I checked, and similar prices for a vodka and mixer.
7) There's one more...ah. ACADEMIA. As people have said, and certainly for medics, a rather competitive lot. This might be a bit unappealing for some, but for those who like "playing the game", as it were, this is one place you should certainly consider.
There's some of my opinions on why you should come to Emma, feel free to applaud/approve/amend/disagree/attack