Per US News and World Report, Harvard and UCB are #2 in math, a hair below MIT and Princeton (http://grad-schools.usnews.rankingsandreviews.com/best-gradu...). Harvard certainly has a very strong reputation in math, including the hardest/most famous first year undergraduate course: https://en.wikipedia.org/wiki/Math_55 (i.e. one that might be attractive to TA for).

Disclaimer: my first job was being a sysadmin and programmer for an applied math group at MIT.