I know its easier said than followed, but there's isn't a thing as a life-deciding exam. From your own assessment of yourself, its obvious that you're a good student and will make it to the school of your choice. Plus you don't seem to have any (major) financial issues either. So as the more experienced forum members have suggested, relax and make a wise decision. Although I am from India, and I know next to nothing about the system elsewhere (:-D) I have seen that quite a few people experience a change of heart during their undergrad phase which would either result in a change in interests or worse, loss of them. IMHO, the more important thing is to choose your major wisely. Isolate your interests and figure out if you're interested in research/you want to do something else after BS. There are some nice 'guides' about careers on this sub-forum...check them out. I suppose you'll be better informed after the freshman year about all this, but if you really do have such a wide choice as MIT/Princeton/Imperial/Yale/Berkeley/etc, then maybe you could do some research on what fields they're good at (but I'd still go with mgb_phys's advice if I were in this situation!). Besides, I really do not think I can figure out which univ is better than the other in some field...I'm told its a practice some students follow, but I don't feel I can ever asssess them this way...it would be a n-th order hyperfine splitting carried out just for the sake of it. However, you could go with labs, facilities, electives, options, etc.
Anyway, to elaborate on the life-deciding part, I'd say you have to pay something to get something in return...if you don't like your education system which defines this "life-deciding exam", then so be it...welcome to the group of individuals who are forced to be a part of a system we don't like ourselves, but we have to go through anyway and are---as students--unable to change ;-). You won't always have to cram stuff, and there will be more palatable things to do eventually...think of this as a road to get there :-) Besides, I don't think your life can be decided by the outcome of one exam, if you're so committed to learning. I'm sure something will work out, provided you're open to a lot of different opportunities, some of which may not be immediately appealing or pleasing.
Best of luck!
[I'm unusually optimistic this morning...I guess I should get some sleep now.]