Why did people say the Warriors were the best team ever when they're not even better than the Thunder?

Because these same feeble-minded people bought into the hype. Let's not forget this team won last year's title 4-2, against an extremely depleted and banged up Cavs team. They come out this year and beat the Bulls's regular season win record. Nice job, but it's just the regular season. This team is almost entirely built on Curry and Thompson hitting three-balls. Sorry, but the rest of the team is fairly average. They simply are. Not awful, but average. If you get those two off their game, then it becomes easy to beat the Warriors. All these muthafuckas getting carried away with their title last year and the regular season wins this year, have sure gone quiet the last week. They know, Curry knows, Thompson knows, Kerr knows, we all phucking know, this team can be beat. There are some scared muthafuckas in Oakland right now.

Because they had the best regular season record (also, they were better than the Thunder during the regular season). The problem is that the regular season isn't a reliable representation of team strength anymore, but people still like to pretend it is.

They broke the record for the most wins in a regular season.