I would guess elves. Because they seem to not get slow and at least look old. Witchers do.

I read somewhere that a Witcher's age is around 400 years. But an elf definitely lives more than a witcher.

elves

(probably Aen Elle even longer than Aen Seidhe, which may be a bit ... degenerated).

Oldest known witcher, Vesemir, may be about 200 years old, but it's not surely cinfirmed. King Auberon (Aen Elle) was more than 650 years old. Probably Avallac'h similarly.

This can also be assumed by the look. For example, Geralt in the games is actually in his late 90s but he looks more like in his early 30s (compared to normal human age). Then if we do the maths, the average age comes around 300-400. This also indicates how Vesimir looks like in his late 60s while he actually is around 200.

For the elves, I have seen a conversation between Geralt and an elf in Witcher 2 where the latter tells Geralt how the elves lives so much longer than the human beings probably more than the Witchers.

But I have a doubt that probably (or maybe) higher vampires like the Unseen Elder have even more age than the elves ( if we are considering them) rather Elves definitely have more age than the Witchers.

Geralt is between 50 and 60 years old,

Well, maybe about 60 in The W3.