Karazhan. I had no idea about dps, but researched all the bosses and had this notebook with the most important points for each I was such an innocent hunter back then. I didn't even have a clue about gear, and never got a tier set together in all of TBC! But I had the most fun.
Molten Core or Zul'Gurub on my brothers hunter.
On my warlock it was vanilla era Onyxia. She definitely deep breathes more if you have bad gear so it was a rough time but we got through it :0
But that was only pve the absolut first raid must have been a relic raid in DAoC (pvp-raid)
Numbers were 100 people upwards ^^ (that was when mmo's still had that first M of mmo for massive)