#2 Willian - Chelsea
The Brazilian has worked with Jose Mourinho before, and the Portuguese manager is a big fan of his game. With Maurizio Sarri still trying to figure out which attacking trio he should go for, it looks likely right now that Willian will miss out. It's not indicative of his ability though, he just doesn't show it often enough with Chelsea.
Although many wouldn't believe this is the case, working under Mourinho could actually benefit Willian. He has shown an eagerness to work with his former manager before, so if the feeling really is mutual, then he will receive a tonne of confidence.
Willian, being raised in Brazil, has a lot of flair to his game. Chelsea fans will argue he doesn't show this enough, but if you watch him closely, then he will often show signs of his natural skills as he aims to work around his opposite number.
At 30, Willian probably does not have a lot of years to offer but he has been playing in the Premier League for 5 years now and he would not need time to adapt to the style of play or new opponents. However, it is questionable how eager Chelsea would be to sell to Manchester United.