Dartmouth ranks as one of the schools with the best overall quality of life. Coupled with the strongest study abroad program in the Ivy League, an exceptional and loyal alumni base, stellar prestigious grad program placement rates, an immense undergraduate focus, and more school spirit/community feel than you will find virtually anywhere else, Dartmouth really is the better choice.
Cornell is much bigger: 14,000 undergraduate students vs 4,200 at Dartmouth and 2100 at Williams. It is also less prestigious, very cutthroat academically, and widely considered one of the hardest Ivy League schools to attend.
Williams has very strong grad school placement rates and strong undergraduate focus - Dartmouth, however, is on par in both those areas, so neither school has a discernible difference. Those two factors are Williams largest selling points, but Dartmouth has them covered.
In addition, Dartmouth has the small school feel while being twice as large as Williams. Therefore, everyone can find their own social group and niche. This includes students who are not inclined to drink or join the Greek scene. Since you are research oriented, Dartmouth is large enough to conduct strong research and small enough to still be extremely undergraduate focused. It’s the embodiment of the best aspects of a research university and a LAC. Dartmouth’s D-Plan also allows you to conduct outside research, study abroad, or do an internship during the school year which gives you an edge over the competition. Neither Williams nor Cornell is designed around such a system.
You have three excellent options for to choose from. Only Williams and Dartmouth, however, are really worth your attention. You cannot go wrong with either choice, but the evidence does seem to be in Dartmouth’s favor.