In terms of academic rigor, Caltech probably has no equal. Part of the reason is everyone there has to take the same physics sequence, regardless of majors. The famous physics problem of toilet paper unrolling while dropping is just one of the examples. Unlike Harvey Mudd or MIT, Caltech is on a quarter system and its courses tend to proceed at a faster pace. Most of its students also take at least 5 full courses each term (quarter).
On the other hand, UChicago doesn’t seem to be as rigorous as many people thought:
http://talk.collegeconfidential.com/university-chicago/2155668-uchicago-core-sequences.html#latest