Baker's likely the safest as he's displayed a solid overall game. Watson can be a good pass catcher (he looked very good in '07), and he's shown some improvement as a blocker over the past few years. Thomas is also a decent receiving option, and he's a better blocker than Smith, as well.
They signed Baker to a 5 year deal, so I don't think hes going anywhere even if he was terrible in preseason. He hasn't been, so hes definitely a lock. That being said, I agree, I just don't see Smith bumping anyone off the roster other than maybe Devree.