In my opinion, the two teams that have the greatest chance of winning the Europa League are Juventus and Sevilla. I would put them even above United, simply because neither of those teams will achieve a CL qualifying place in Serie A/LaLiga, so they have to win the UEL if they are to play in the CL next season. Their motivation to win this tournament is much higher than for example United.
Even though Juventus and Sevilla are unlikely to qualify for the Champions League in their respective leagues next season, they must demonstrate in the Europa League that they can compete for the league title this season. The fact that they did will not qualify for the Champions League does not guarantee that they will show the same expertise and zeal in the field of play to win the Europa league title