I think there are different measures.
Cost versus return would have to be Soldado, but he's really tried and has had a difficult time of things.
Most underwhelming purchase would be Paulinho.
Least effective use of talent and passion would be Vertonghen.
Weirdest turnaround in form (from good to useless) would probably be either Dembélé or Sandro.
Overall, though, I'd be tempted to say Lennon or Rose, but I'll go with Lennon on the basis that he should be performing better.
I'll be honest in the fact that I think it's 99% tactics and training that have contributed, rather than talent, although effort could have been higher from some.
Again it's a ability verses attitude, what ever the bad management of avb or Tim Sandro, verts and dembele's attitude has quite frankly been shit, when a 19 year old has more balls than the 3 of them combined it makes my blood boil.
I almost feel they owe the fans an apology, when naughton makes a mistake it's because of who he is you can't blame him he's trying, what's excuse do our better players have. All of the then need to look at eriksen and follow his lead.