Alex & Izzie
When I watched GA for the first time, I really loved Alex & Izzie. I liked their chemistry, their build-up, their friendship and their relationship and I still do. But after rewatching GA, I realised that -in my opinion- Izzie never loved Alex the way she loved Denny. I think she's always loved Denny more than Alex. I do believe she really loved Alex, but even when they were in a relationship in season 5, she was (mentally) cheating on Alex with Denny. (And I really love Alex & Jo together)

Tyler & Caroline
I started shipping Forwood in season 2 of the TVD. I really love their dynamic and their relationship in season 2. The way Caroline takes care of Tyler, the way she helps him when he turns into a werewolf, those moments were just great. They had a great build-up. I was really happy when they became a couple in season 3. The problem I have now with Forwood, is that the writers totally screwed them up. I really dislike this whole Klaus and Haley situation (I stopped watching TVD halfway season 4)

And of course I mean 'much instead of 'luch', but I can't the typo anymore

I used to ship Forwood so much but now I want Caroline to be with Klaus! Arrrgh! Why Caroline is so shippable ??!! :DD I still love them a lot though
Well Izzie/Alex always had been my fav couple on GA and the fact that she left in S6 was always a part of why I stopped watching the show. But I kinda agree with you on the whole "Izzie mentally cheating on Alex" :/

I don't watch TVD, so I don't know.

As for Alex/Izzie I used to ship them so hard, but then there was a time where I didn't like them as much (When Izzie left Alex) but after rewatching the show I started shipping them even harder and I absolutely adore them.

Alex/Izzie are my favorite couple on GA and that won't never change !

As for Tyler/Caroline I agree with you and Aline. I shipped them hard in season 2 and the beginning of season 3. But now I definitely want her with Klaus without no doubt! Forwood was totally screwed.