I'd say Mercedes but apparently they're not friends anymore, I'm also not picking Rachel because she treats him like crap (still waiting for an apology for the whole BIOTA incident.)

^SAME.

Rachel ran against Kurt after saying that she'd help him and stuff just to add being student body president to her application for NYADA. Then she stuffed the ballot boxes for him, and got suspended herself (which is what she probably gets for doing something like that anyway). And Kurt didn't even win anyway. That wouldn't count as being a good friend to him. Certainly didn't help at all.

Mercedes and Kurt used to be friends. But then she got pissed at Rachel (I agree with everything she's upset about) and joined The Troubletones, and started bashing at New Directions with Santana, Brittany, and the other members. So I don't really see them as best friends, regardless if Mercedes rejoined New Directions or not.

Although they're boyfriend and boyfriend, Kurt and Blaine are friends no matter what. They stick up for each other (well, mostly Blaine sticking up for Kurt in the second season) and spend a lot of time together. So...yeah, Blaine FTW.

@mangie1995, what does BIOTA mean?

Blaine is his BF, not his friend. I miss kurtcedes:(

Finn understands Kurt in a way that no one else does and even though we haven't seen much of it lately they share a connection stronger than blood. I'd also say Mercedes, she's probably one of his best and most trusted friends even if he's seem to have been giving her the cold shoulder these days. Really it's a HUGE tragedy that RIB no longer allows Kurt to have these two wonderful people back in his life when they were the ones there for him first, especially when he was struggling with being honest with himself. Nope, instead RIB has shamelessly exchanged the Furt/Kurtcedes friendships in favor of (no ofense) lame Klaine and blah Hummelberry! Kurt no longer has a life outside these two people, he might as well be sewen onto both of them or live in a bubble with them for the rest of his life:/ SCREW YOU RYAN MURPHY YOU IGNORANT, HEARTLESS BASTARD!!!!! BTW Rachel is not a friend to him, she's betrayed him, back stabbed him and basically treated him like garbage! He's her competition and always will be which is why their friendship won't last!