I'd argue that after Jim & Pam got Married, that would've been a fitting conclusion since the show did center around these 2 getting together along with Michael for the ride.. Season 2 & 4 are the Best and Season 5 with the Michael Scott Paper Company episode with Idris Elba was Excellent
shareI would say season 7 after Michael left. Michael was the heart and soul of the show, and it struggled to find a new identity after he left, and it never really found it.
shareEnding with Jim interrupting Pam's talking head to ask her out at the end of season three, and Pam's look of undiluted happiness to the camera immediately afterwards, would have been something - leaving their 'happily ever after' to the viewer's imagination.
However, as other people have said, series 5 would maybe have been a better spot to finish on. In my opinion, all of 4 and the first half or 2 thirds of season 5 are really strong. I'd try to wrap it up there though, and not carry on to 7 when Michael left - In addition to (my opinion again) the quality going downhill towards the end of series 5, Pam just isn't the same when she becomes a salesperson and Jim also becomes more and more unlikable as the show goes on, can't quite place why. They even try to re-hash the Jim and Pam romance with Andy and Erin and it just doesn't work.
So maybe if Jim and Pam's wedding had somehow been brought forwards a series, they could have had Michael move away with Holly (which is what they ended up doing 2 years later anyway) and ending it there.