As far as Tony Vargas' claim that the "best" system has to be universal......I don't know how I feel about that.
I hope I made it clear that I was talking about a specific, personal, definition of 'best,' for the sake of trying to answer the question.
The reasoning is on several levels: for one, a universal system can always be compared to another system, it can't hide behind "that's not what I do." For another, you only have to adopt one 'best' system if it's universal.
But, mainly, I just find the concept of a universal system appealing, much like a Grand Unified Theory in physics.
I think you could argue that the best RPG is one that accomplishes what it sets out to do the best.
What about the RPG that sets out to be the worst RPG possible and succeeds? ;P
If rules-heavy systems don't give you a game experience you like, claiming that something like GURPS is the world's greatest system isn't going to resonate.
I'd draw a distinction between 'best' and 'most popular.' Even between 'best' and 'most fun.' Put those two together and it puts the importance of theoretically 'best system' in perspective. It's really virtually moot which game is 'best,' it's interesting to think about and analyze, but there are so many subjective factors that affect the play experience in a TTRPG, that even if you could isolate the 'best system,' it wouldn't guarantee anyone would buy it or enjoy playing it.
...
And, in the context of this thread, another reason to consider universal systems:
But we don't know the genre or style beyond that, so we can't usefully answer the question.
Or, you can factor genre out by considering universal systems.