I really like his behaviour in 'Of Shadows and Skulls' when he bosses the Horde around, but then bricks it when he realises Hordak isn't around to protect him. When Hordak returns, I love Imp's line "...have I got some stories to tell you! Better get that old trap-door ready!"
Also, in 'Loo-Kee Lends a Hand' it's great when he says to Hordak "Can I drop him down the trap door? Can I? Can I?" (about Mantenna).