As we have seen lately, the Death's Shadow decks are packing a lot of punch and seeing a lot of success. Infect, on the other hand, has seen a big drop. I think the metagame is too hostile for Infect right now, with lots of discard, removal, and Collective Brutality (which combines both!). Affinity is still a tried and true solid Modern deck that you can't really go wrong with, unless people are packing a ton of SB hate. So probably Shadow, then Affinity, then long gap, then Infect. Shadow is both hard to play with and hard to play against, so if you get in the reps, it will pay off.
As for Mox Opal, it seems like a card that could be banned. But who knows. It's very strong, but not currently warping the metagame. There may have been a case if the Cheerios deck ended up being more consistent and less fragile. So right now, I think it is safe. However, it might restrict design space a bit because they need to worry about printing good artifacts that go well with it.