IMO there is no 'best' sim. There is only the sim that fits your individual needs the best. For some that may be iRacing, for others it's GTR2, rFactor or something else. They all offer things that the others don't. It's just a question of what you want and the kind of environment you like to race in.
Well said, for people like me various games are actually "best sim" for different things and moods I see no reason to bash someone else's best sim:
* iRacing - best online racing and "pickup" races that feel like league races since I don't have time for a league
* rF/GTREvo/Race07 - best variety of fan made racing mods and tracks that are unique and don't exist in other official sims
* nKPro - best sim with great physics and FF that also supports rain, great track mods and converted tracks from rF/Race07 etc (Macau is my favorite)
* GT5 - best Nordschelife (and now Spa) hotlapping sim with endurance type 24hr weather and changing daylight/night effects
* FM4 - best car collection and car career sim
* RBR - best rally sim
* SBK 2011 - best motorcycle sim
For me there are lots of "best sims" and I am just glad that we have all these quality options available at reasonable expense that I can afford, we've sure come a long way from the days where I caught the simracing bug on the C-64 and then PC.