Runescape around when it first started probably.
My first full client game was probably maplestory beta or ragnarok and ROSE.
EDIT- Oh Shoot, I forgot about Survival Project. That was a fun game, even if it was pretty pay2win, I usually beat the cash shoppers. Too bad the English version got shut down, but I heard of someone trying to make a private server for those who can't access the Korean one. I think it's called "spgame.net" or something. I'll have to check that out.
You know, probably instead of just posting the first MMO we've ever played, we should probably post a timeline of all the MMOs we've ever played since the start and for how long.