Email, as far as im aware there isn't some alternative email standard (messaging services, whatsapp, signal, sms, etc do not count imo as I believe they serve a different purpose than email)
DNS, while there are alternative root servers, they still fundamentally rely on the dns protocol.
TCP/IP, when the internet was first starting, this was not the only standard in use, but now it is (to my knowledge).
I thought about this for longer than I should've for a comment on a random post, but this is all I could think of lol.
edit: grammar
Ah, I shouldve been more clear. I didnt just mean tcp specifically, I meant IP as a whole, for an example of a competing standard see x.25.
Funny enough, that wikipedia article mentions that x.25 is still in use by the aviation industry, and after a quick search it seems it is! So I guess Im still wrong lol.