Theymos probably has the highest activity here, there were only 34 people registered before him, his uid is 35. But possibly there are few older accounts still around. There's
SmokeTooMuch, registered Dec 2009, last active 3 weeks ago, but his last post is from Jan 2016, so hard to say whether he's still active.
Wow, his user id is 13. It would be interesting to find out as well how much accounts so ancient might cost. Not that I'm particularly interested in buying one (I'm quite happy with what I have), but I gather that they would cost quite a lot, especially those with a post history which made them into heroes or legendaries.
Here's the account with user id 1. He didn't post much, though. Still a newbie, lol...
By the way, Satoshi is
third (makes an interesting read)