I train in Muay Thai but understand that it would be smart to learn grappling, which ones are best with MT?

bjj and wrestling are the best ones. bjj is an improved brazilian version of japanese jujitsu, in judo you learn trows which work only if you perfect them, and dont know much about sambo. bjj is good for chokes and submisions but since you want to defend, start with wrestling, so you could defend the takedowns. lf an opponent cant take you down he cant submit you unless he uses flying armbar or standing guilotine so its best to learn both bjj and wrestling