<p>I'd say that Princeton and Dartmouth are the most undergraduate-focused Ivies. Both have great bio programs and on-campus research opportunities, but Princeton's on-campus research is better because they have a larger graduate program. Dartmouth, on the other hand, can offer better internship opportunities because of the D-plan.
Some info about Princeton's bio dep: <a href="http://www.eeb.princeton.edu/UNDERGRAD/Intro.html%5B/url%5D">http://www.eeb.princeton.edu/UNDERGRAD/Intro.html</a> .
Dartmouth: <a href="http://www.dartmouth.edu/%7Ebiology/undergrad/%5B/url%5D">http://www.dartmouth.edu/~biology/undergrad/</a> . If you graduating with a bio major from either school, you'll have very high chance of getting into the med school of your choice.</p>
<p>Columbia is very strong in the sciences, but it has WAY too many grad students for it to be very undergraduate focused. I'm sure you could fight for some opportunities there, and do great, but it might be more difficult. I wouldn't say Yale is terrible in sciences; it just isn't as known for its sciences compared to some of its other programs. I don't know as much about the big H, so I'll leave that to someone else.</p>