On form and merit this season:
1. Rafinha
2. Denis
3. Arda (no control, better as a FW as others have mentioned)
Denis tends to drift wide a lot which is fine, moreso on the right where Messi is since he naturally tucks in but on the left AM role, he needs to be more central that wide since we already have Digne/Alba and Neymar who like the space out there as well. If Denis is continually wide, we lose a passing option through the center and that can imbalance the formation. At this exact time, I think Rafinha would be slightly better in understanding the positional demands.
But that's based on what I've seen from them all this season and particularly Rafinha and Denis need to get more time to really see how they are progressing so, much like last season when Messi went down and Munir and Sandro got chances to show something, now Denis and Rafinha can do the same with Iniesta's spot.
Gomes game against Valencia was worrying. He was struggling to with the pace of the game which was surprising to me based on his pace. Maybe it was playing against his former club that threw him off a bit but it's not the first time he's struggled like this. Best as an Raki/Busi sub.