Barcelona are currently in their best form. They are more likely and favorite to win the Europa league title. The only two competitor I see that would challenge Barcelona to the finals are RB Leipzig and westham United. Westham are the only English side to be among the Europa qualifier till this moment, after the beat Sevilla in the extra time of the game. Let's wait and see the Europa league draw fixtures before we conclude, but I'm hoping for Lyon or Atalanta to host Barcelona.
I don't know why you think that Leipzig will make it difficult for Barcelona if they meet in the quarter-finals, but I agree about West Ham. So far Leipzig have qualified automatically for the quarter-finals after their match was canceled by FIFA, so you really can't be sure that Leipzig really deserved to qualify if FIFA didn't cancel their match against Spartak Moscow.
Apart from West Ham, I think Atalanta are also a strong team so far both in the Europa League and in their domestic league. Atalanta may have a chance of qualifying for the semifinals of the Europa League this season despite having to meet Barcelona, but it will certainly be difficult. I hope barcelona can reach the final and win the title this season because of their high hopes so far.
Leipzig may not have played against Moscow and automatically qualified, but they are a much stronger team than Moscow and most likely would have qualified.
The current favorites of the bookmakers are:
Barcelona 3.5
Leipzig 5.5
Atalanta 9.0
Lyon 13.0
West Ham 15.0
Barcelona and Leipzig seem to be the strongest teams by a margin here and it is probable that at least one of them will make it to the final. Of course we are in the K.O. rounds and many things can happen, but whoever plays against any of these two is an outsider for winning.