Danario alexander is, in my opinion, the definite WR flex. With Carolina's offense going crazy lately, San Diego might need to pick up on the throwing game to keep it even. Couple that with the fact that Danario has put together 4 double-digit games in the last 5, and you got yourself a very solid flex.
Moreno on the other hand, has a very enticing matchup vs. Baltimore. There are a few downsides I see to this however. For one, yes, Ray Lewis is quite possibly returning. And for two, Baltimore just got a new offensive coordinator. This could mean many things. For example, this new guy could make Baltimore's offense very explosive and unpredictable in its first debut under Jim Caldwell. That could possibly leave Manning throwing a lot while Moreno gets less chances. That being said, that scenario is filled with what-ifs.
Boldin has some upside, but the Broncos defense is unpredictable as of late, so I just don't know. Also an enticing choice, but Alexander beats him out.
Cecil Shorts is filled with questions of injury and with an unreliable QB, its safe to bench him for the week.
So with that, I say choose Alexander. I believe he has the most upside overall.