Panty and Stocking is the series I've dropped the fastest ever, 10 minutes it took, utter garbage any way you slice it. Tried it because I'd heard the ED, witch is a great song.
It will be Garo: GnT when it finally fucking finishes. It should not have taken 17-18 episodes to finally become interesting. I was so absorbed into ep 18 that I lost track of time. Stuff was finally happening that I could care about.
Oreimo, K-On or Chuu2 if 'completion' means finishing first season before s2 was a thing.