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.
Don't think Source ever won out, Global Offensive was the game to dethrone 1.6. At least in our "semi-professional" neck of the woods CS:S adoption was 30% at best. A clear majority were still playing 1.6 even when Global Offensive came out, and even then it took a year or two of updates to make that game good enough to start convincing significant portions of people to switch over.
For example in 2011 Electronic Sports World Cup just before Global Offensive came out 1.6 still had a significantly larger price pool than CS:S. Funnily it was also the first and last ESWC to even have a CS:S tournament, even though the game had been out for 7 years at that point.