TBC is where I started raiding but I had some misfortunes in finding a good raiding guild. My first guild was so bad it could hardly progress after Attumen, it was a guild that was made by some friends that grew to be a casual guild but wanted to grow to a raiding guild. That never went anywhere until we started to ask good people to help us out. My second guild was a lot better that had Kara on farm, if the group was good we could clear the whole place in one evening. This guild eventually died out due to a lot of inactive members and the raid leader not willing to form raids anymore. My 3rd guild which i'm still in today was pretty good who also had connections with some of the best guilds on my server, we formed several raids merged with different guilds and had Hyjal and Black Temple on farm. I didn't get far enough to raid Sunwell because of how exclusive the place was to only the most hardcore guilds, and I was not hardcore enough to participate.
In WotLK I had the best raiding experience with my guild. Then in Cata I was burned out of raiding and wanted to just play the game casually.