To me, any actor playing Strange needs to be able to pull off the following:
a) Supreme competence, whether it's in the OT or casting crazy spells.
b) Extraordinary intelligence, but a quiet, contemplative type of intelligence.
c) Confidence, bordering on overconfidence.
d) Something intangibly otherworldly.
Phoenix would've been my top choice, but that ship has sailed, and realistically, I think most of us knew that he wouldn't commit to a contract that had him locked down for 6 or so movies.
Ewan McGregor checks all four boxes for me and is my current top choice. I still consider his turn as Mark Renton in Trainspotting as an all time great performance. Doesn't always pick the greatest of movies, but in terms of pure talent, I don't think you could do much better.
I'd be cool with Hawke, Pascal (though I'm not sure if he was rumored or merely fancast) or Oscar Isaac.
Cumberbatch, Leto, Gyllenhaal and Tom Hardy are all somewhere between really good and fantastic as actors, but for some reason, I can't see any of them as Strange. Then again, wouldn't be surprised at all if one of them gets the call and subsequently murders the role.
Reeves gets a big "NOOOOOO!" from me. I just cannot see this work from any frickin' angle. Let's just move on.
Farrell is an interesting case. He's done fantastic work in some indy fare... he's spectacular in In Bruges, for example. But he's been absolutely horrible in some other parts. Miami Vice (dude was coked up during the entire shoot and it shows) and Daredevil (though in his defense, everyone in that era of CBMs were prone to hamming it up) come to mind immediately. Plus, he comes with quite a bit of baggage... though nothing insurmountable. Overall, if I were making the decision, I'd consider Farrell too much of a risk.