Someone mentioned Northwestern, ignoring the weather. If visiting, coldest month is Jan. Worst month is March because you just want winter to be over! Spring runs from May 10-17.
My experience may be broader than that of many posters as I hold degrees from 5 schools, have taught at 5 others, and interviewed for positions at dozens of schools. Although I am east coast born and bred (and have had affiliations with Harvard and Columbia), it is hard to beat Stanford and Berkeley. UCLA and Pomona also seem to fit the OP’s stated criteria. In the interests of full disclosure I hold a degree from Berkeley and have taught there, and my D is currently at Stanford. Those prejudices notwithstanding, I must admit that I shared many of OP’s stated criteria when I did my own searches.
No one has mentioned schools outside the US and OP’s criteria did not preclude the consideration of foreign unis; therefore, I would add that Cambridge also fits OP’s criteria, if one does not mind a little rain. The climate is mild year round, and the town is picture perfect.
^^Now I’m curious to see which school you would rank as the least-perfect, given the parameters.
@2muchquan I think you are exagerating. We visited U of Chicago in mid April and it was gorgeous. I can’t imagine it was that different half an hour north!
My uncle lived in Evanston - and I remember the weather at Thanksgiving always being horrible. Cold and windy.
I like Oxford better than Cambridge. 
(Lord Peter Whimsey and all that!)
@Prof99 McGill (a foreign uni) was mentioned way back when weather was temporarily removed from consideration.
@mathmom I was not exaggerating. Much. You lucked out when we had that one nice day. It was in the news.
“Spring runs from May 10-17.”
Gonna have to meet you one day.
UNC Wilmington. 10 minutes from the beach, wonderfully chill atmosphere and students, great sports and school-spirit (for mid-major standards), terrific academics, top 10 value public school, perfect medium size, gorgeous student body. Probably the most underrated school in the country; I’d venture to say that it would be nationally-ranked on USNWR if it had football (it would draw more applicants from around the country).
And, they’re also blessed with my presence. More than enough reason to move it up. 
[QUOTE=
@Waiting2exhale
Love to, but my spring fills up fast! :))