Which Ivy League School has the prettiest campus?

I like Dartmouth 

Princeton and Cornell would be my top 2.

I attended Yale and Penn, and my husband worked for Princeton. We lived in Princeton for over 10 years, just blocks from the campus.

Princeton is generally considered to have the prettiest campus, and I'd say it does, but frankly, it's one of the smallest of the Ivies, and the town is quasi-suburban -- really oriented to affluent families rather than students. After a while, the place becomes stale, though the fact that you can pretty easily get into New York and Philadelphia via the campus train helps.

My husband once described the town as a "giant Crabtree and Evelyn" store.  That wasn't inapposite.

I've visited all 8 for one reason or another.  I have to say I'm fondest of the Yale environment, though it's really the grittiest urban campus of the bunch.  There's simply something rather exciting about it, even more so than the campuses of Columbia and Harvard.  Being a student there was the most sparkling, stimulating, intellectual experience I've ever had.  It changed my life.

Princeton, Cornell, and Dartmouth would be my top campus picks.  This certainly shouldn't be what determines where you attend though.

I attended Cornell, and have been to all 8 Ivy campuses. I am of course partial to my alma mater, but really all of them have very nice campuses. As another notes, Princeton is especially nice. I think Dartmouth is very nice as well. Penn, Harvard, Columbia, Yale, and Brown are city campuses, so they have quite a different look than the others, but they each have a lot of nice historical buildings. Cornell, which sits on top of a hill, definitely has the nicest view of the surrounding landscape, from campus. 

I've visited Harvard, Penn, Princeton, Cornell, and Columbia (I am a graduate of one of them; not telling which!). I've never seen Yale, Brown or Dartmouth. 

Overall, I think Princeton is the prettiest (and is the standard against which I judge the prettiness of all other college campuses, Ivy or not). What the campus looks like, however, should be your very last consideration on choosing a college.  

We visited four out of the eight Ivy League schools (Harvard, Yale, Princeton and Penn). We liked Princeton the best (it also had friendly students). Yale had impressive architecture, but a more formal feel to the campus. Cornell is supposed to be lovely, but it has a reputation as a high-stress campus. (A friend's son committed suicide while he was in grad school there.)

I've visited the campuses of six out of eight.

Most of them are actually quite "urban," and might be architecturally striking, but not what I imagine you mean by "pretty."

Of those six, I would say the "prettiest" one is Dartmouth.

Note - I have never seen the campus of either UPenn or Cornell.

That depends how you feel about snow. Do you really mean "Ivy League" as in the 9 schools in the athletic league? Or are you using it generically, as many people do, to mean elite universities? If the second, I think Duke and Stanford have the prettiest campuses.