Erick Pieters. I buy him every save because almost every team I play lacks a backup DL. He's cheap, doesn't have the best stats, has low star reports, but for some strange reason he always plays well! Not like ultra-goal-scoring-well but he rarely plays bad or makes mistakes, which is excellent for me as a backup. He also doesn't complain about lack of playing during the first season in.
I buy him first season at 2.5M€-3M€ and you can easily sell him at 7.5M€-12M€ at the end of the season, depending on your team's reputation.