I love their friendship.

Why?
"The Suitcase" answers it. But even before that, their interactions were always interesting.

and then Don&Peggy friendship

He's like her big brother and she's like his little sister. Their relationship is beautiful. Don actually trust her, a rare feeling for him. Peggy is like fascinated by him. They have mutual respect for eachother. Closest friends! No romance, this is deeper. For example, the episode where Don gets the call that Anna died. Peggy and him stay up all night. So amazing!

I love Don and Peggy AS FRIENDS and COWORKERS!!! They should not ever be more than that, ew! It'd be like a brother-sister thing! I don't like Roger with any woman, 'cuz fuck, dude make up your mind!!!! And, as pissed off as I could get at Don for all his philandering with Betty, she is a fucking BITCH who has handled the divorce like a petty, angry child! She treats her kids like shit, and everyone sees it, and why???? I officially hate her now, and she handled the situation extremely poorly! So, no, not her and Don, even tho I do think she still has feelings, and I hope Henry leaves her! As for Peggy, and Pete.....far too fucked up, and actually, him and Trudy have really come a long way. They became a real couple, when i didn't think they'd last at all at first. I love Peggy, but I kind'a want to see her find a nice guy she can confide in instead of sleeping around.

^ I picked Don&Peggy not because I want them to be together romantically, but because I love the relationship they have. Friends, coworkers, they are there for each other, they trust each other, and their interactions are the best.

Romantically if I have to chose I don't really have any favorites, I would chose either "none" or Roger&Joan I like their scenes, and as a couple they are interesting to watch, I like them and their story.

Also Joan & Roger and Peggy & Don (as friends).