My memory is terrible, and it doesn't help that I have the world record for the most sites having signed up for (a very nerdy bragging right I have), but it has to be something like one of the Pokemon MMO's, RuneScape, or Furaffinity (RIP Dragoneer). Most of the old places I've been to are gone one way or another, for example I haven't heard from Postopia since the E3 disaster.

iTools > .Mac > MobileMe > iCloud. Still on the original address. January 2000.

My proboards account, made in 2004.

Yahoomail account. Created it in the early 2000s, stopped using it around 2009 when I moved on to newer email providers. I remembered I still had it last year when I got an emai on my recovery email that they will be deleting my account if I don’t login. I somehow was still able to recover it and login after 15 years. Now I just login whenever I get a notification of inactivity. I don’t actually use it for anything anymore.

My Twitter account is from 2014 and I still use the same one. My youtube account is even older but no idea how old. On places like Reddit and Lemmy I make a new account atleast once a year.

YouTube! I've been using the same account since 2008. They actually had to makeup an email for it since my account was before the Google merge.

i believe my neopets account is older than my youtube account, so that. actually went to playing neopets again fairly recently to see how its changed

Just missed my ICQ account which was about 27ish years old. Ok it wasn't in frequent use.

My gmail account has been in continuous use since 2009, although I use it less now that I used to.

Bank account is first, use it everyday. Second would be apple id.

Lots of people mentioning Neopets. I did have an account there but couldn't log back on when I tried a few years back, so I guess my account was deleted (or I've got my login details wrong)

I think a spare email I use for peripheral stuff is from mid-late 1990s. Maybe 1996.

Still active. Probably Steam.