Agreed on The Passenger
. Sid didn't do "possessed" very well I don't think. He was equally stilted as the Changeling Bashirs in both The Adversary
and in Purgatorys/Infernos
That said, I have to say I LOVED Bashir in both the early and in the later seasons, and, I know I'm in the minority here, but I actually think the retcon *worked* really well for the character.
In the early seasons, knowing how dark and gritty the station and the crew was, and seeing doe-eyed Bashir come on I thought "This guy is going to get eaten alive!" To see him around O'Brien and Kira in those early seasons made me wince, but having that conflict between the characters is what the show was about. To have them all grow closer to one another over the years was one of the reasons the show is so endearing to the fans I think.
For me, the game-changer for Bashir was Distant Voices
. Not only was this an excellent episode, with some top-notch acting from Siddig, but for me this was the bridge that connected early season Bashir to the more confident Bashir of the later seasons.
In that episode we get to see what's inside Bashir's head as the Lethian calls Bashir out on several issues, telling him "There's no secrets in here". In that context, you understand that the early, seemingly naive Bashir was a total con. Sure the writers didn't know the genetically enhanced angle yet, but still - the intentional flubbing of his entrance exams, the fact that his Dad discouraged him from becoming a tennis pro, the nervousness about the attention of getting the Carrington award - it actually works. It's not a perfect retcon of course, but it's one I'm willing to go with. Honestly, after the retcon was revealed, it actually made me like early-Bashir even more.