Easily BoB 2. It has the best FM, AI, campaign and general immersion of any WW2 sim I've flown.
It's limited in scope, only a few flyables, one theater and it's somewhat dated but, on a qualitative scale, it outstrips it's competitors from the last decade in its present form. I mean you can't beat the sheer detail in BoB, the aircraft, campaign, and the AI are all absolutely superb and the game world is full of wonderful little touches that take you back in time.
Post V.4.11m Il-2 1946 comes a very close second, it's rather more sterile than BoB overall but the new AI and engine tolerances have pushed it out of the twilight between game and sim where was languishing before TD took the reins. It still lacks the complexity of a true sim in many respects, generic and simple engine management and flight models, but this is constantly changing under the auspices of Team Daidelos (further augmented very dedicated and talented modders) who are steering it in a promising direction.
It's only by the tiniest of margins that BoB has it's nose out in front but I think Il-2 will outlive it.