I havent used Diamond Lillard but a couple of times and he was the best player on the court.
There are a mass amount of undervalued legends. Wont even begin to name because I have used a lot of them and they are dirt cheap. I will say...I picked up Dumars today for 5k.....I can almost bet he is undervalued.....can play both guard spots, defend, and score. That young Pip card is great too. I think I got mine for 2500...but you can easily get for 3500 or less.
Off the top of my head though.....I would have to say Ruby Magic and Gold Lebron are the most undervalued players in the game.