When I was a kid here in the US "thong" was the only name for the type of footwear. I don't know when "flip-flop" came into the language, but it kind of sneaked up on me.
As for the underwear, I don't think there is a "comfortable brand". I've got a couple of pairs, but I don't really find it comfortable to have a string pulled tightly up my butt crack. I'll admit they do help a bit with tucking, though.
Lyric