I like Sid, just not Sid & Riddhima. Sid is allowing AR to meet, he isn't selfish.
Armaan - let's not start on his long list of sacrifices, just know there's a lot. As you said, he left for Riddhima's happiness. He wasn't mentally stable and thought she deserved better; again not selfish.
Now Riddhima.. I don't know if she's selfish or not. If she was selfish, she wouldn't have married Sid. She married Sid because of society and family. Yes, she wanted to suicide, that's where the selfishness comes in. But Sid saved her, I guess. Thing is, she gave consent to this marriage even though there was force involved. Not selfish.
I don't think any of the characters are selfish, and I feel sorry for every single one of them. Armaan & Riddhima were always meant to be. Armaan doesn't know SR is married, Riddhima is confused between the two, and Sid is confused between AR's love and his.. liking(?).
All these characters seem so mahaan to me. Makes me wonder how realistic the show really is. LOL.