In terms of worst ever, WoW. My net would drop every 10 minutes on the dot during certain time periods. It would drop completely and reconnect in the space of about 5 seconds, just enough for my connection to the WoW server to cut so I'd have to spend about 5 minutes waiting for it to drop me to the character screen and then trying to sign back in. It made anything worthwhile in that game - PvP, dungeons, raiding - completely impossible for me for a while.
Recently, it's been with League of Legends. I'm now at Uni in shitty university housing, with shitty university housing internet. In peak times my MS goes from 350 (which is as good as it gets) to 4000.