CS before it went downhill. Played so much de_dust (before de_dust2 became popular) from the beta days to 1.6.
When did it go downhill? I thought CS: Source was great fun. Didn't really care for CS:Go
Source wasn't well received by a portion of the community. The community bisected for a while with some folks staying on 1.6. I don't recall everything behind the 20+ year old drama but it was a thing. Source eventually won out and the rest is history.