Which test score(s) should I send to Yale?

Your old SAT score of 2100 is low for Yale, especially for an Asian applicant: http://www.latimes.com/local/california/la-me-adv-asian-race-tutoring-20150222-story.html

Therefore I would recommend “hiding” the 2100 score by NOT reporting any of your SAT scores (including the 1520). Also, as Yale requires the new SAT with WRITING, so your 1520 SAT does NOT meet Yale’s requirements, and Admissions will ignore the 1520 score: http://admissions.yale.edu/standardized-testing

Therefore, your only viable choice for Yale (and HPSM et al) is to report the 33 ACT and submit SAT Subject tests (which you can use score-choice for, hiding your SAT tests).

FWIW: a 33 ACT is about a 1500-1510 on the new SAT, so the scores are almost equal. Keep in mind that admission to Yale (and other selective schools) is not a meritocracy, meaning students with higher scores do not always get accepted over applicants with lower scores. I’ve posted this several times, but here it is again