I like the episode where he is trying to mould Penny with positive reinforcement. He gives her a chocolate, every time she does something he deems favourable. When Leonard tells him how awful his behaviour is, he squirts him with water, saying "bad Leonard". That cracked me up.
I also like this exchange between Sheldon and his mother:
Sheldon: Theyre not my friends.
Mrs Cooper: All right. If you recall, when you were little, we sat right here at this very spot and we talked about some of the problems you had getting along with the neighbour kids.
Sheldon: That was different. They were threatened by my intelligence and too stupid to know thats why they hated me.
Lol. I love that episode and the one where Raj convinces Sheldon to come out with a Green Lantern lamp and they score. I also love this exchange.
Beverley: Oh, speaking of fathers, Leonard, that reminds me, Im divorcing yours.
Leonard: What?
Beverley: Yes. He was cheating on me.
Leonard: When did this happen?
Beverley: Mmm, well, lets see. Sheldon, when did I leave Leonards father?
Sheldon: September 22nd.
Beverley: Oh, yes, thats right. The weekend after Leonards dog died.
Leonard: Mitzys dead?
Sheldon: She was old and blind, Leonard. What choice did we have?
Leonard: I dont believe this. Why am I the last to know?
Beverley: Excuse me, Leonard, I am the one whos getting a divorce, Mitzy is the one who is dead. Why are you the one making a fuss?
Just the fact Sheldon and Beverly are complete sociopaths and bessies.