Honestly I still see Arsenal as the biggest favourite.
They have already proven how serious they are by their effort in the group stage. They don't just care about the Premier League title now. They are caring about the Europa League quite much as well. No matter how many strong teams coming from the Champions League, Arsenal are still the biggest favourite in my opinion. By the way, bookmakers are also thinking the same actually. However they have the same odd with Barcelona for the title which is 5.00. A finale like Barcelona - Arsenal would be really lovely.
It's not wrong to put Arsenal as favourites, but you shouldn't forget the other teams. This season's Europa League gives a slightly different sensation from previous seasons, in the round of 16 there will be many strong teams competing for trophies, the reason being that there are more top teams that have dropped from the Champions League to the Europa League.
In addition to Arsenal, Manchester United and Roma, there are many top teams from the Champions League who will enliven the Europa League knockout phase. Teams like Barcelona, Juventus, Ajax Amsterdam, and Sevilla will enter the ranks as favourites.
Apart from the teams above, we must not forget the teams who are already used to the atmosphere of the Europa League, such as Real Betis, Union Berlin, and other teams. Arsenal have performed well this season in the Europa League, but there is no guarantee that Arsenal can go further easily because there are still other strong teams that will stand in the way of Arsenal.