It's a very subjective opinion, what is the best starter in Eels. Some say it's Mewtwo. Some dislike the fact it's a free two prizes T3 or so. It can get T2 120 (DCE) 40 firs turn. Thundurus is nice, but it always has issues with space. T2 80 isn't so powerful of you can't even get it though, T1 Darkrai D Claw, however unlikely and situational that is. There are just better ways of dealing damage and discarding energy T2 (Junk Arm, T2 Zekrom, Ultra Ball, Juniper).
Smeargle WAS the best starter in all decks including ZekEels. It's gone now though, so the best starters are Sableye or Mewtwo. Thundurus is really an outclassed card.