Best, most accurate benchmarks? Those are not going to be the most practical or even something the average user can do.
For gaming, usually running the game itself is the best way to determine how a particular PC will run a game, as each game behaves differently.
Video editing? Scripted runs using the editor software of choice.
Otherwise, you can go with software that many people use and try to make sense of comparing results. There are so many different software and hardware combinations, including the same base hardware with different firmware and BIOS revisions, comparing results is about as far from scientific as you can get, but you can still get a feel for general patterns.
Userbenchmark
3DMark
SiSoftware...