Hmm... difficult question.
season 1: it was fun to really get to know all the characters and see how they slowly built up their relationships. I liked that. But the season probably was to short to be my favourite.
season 2: I loved Addison, but hated the MerDer drama that she brought with her. George and Meredith had sex... not one of my favourite moments! The Denny-storyline was touching and the episodes surrounding the bomb were awesome! And of course there were the first appearances of Callie and Mark!
season 3: Meredith having to decide between Finn and Derek was BAD. I totally hated that storyline. The Cristina-Burke storyline was okay. I loved the season finale when Burke left Cristina at the church. That was one really moving episode! I hated how Izzie got in the way of Callie and George and how George treated Callie. He should have been honest with her. She really didn't deserve any of that. I loved the "relationship" between Alex and Addison. It was a shame they didn't build on that chemistry between them! I also liked how Alex cared for Rebecca/Ava. He was really growing in that season!
season 4: Contrary to many others, I really liked Erica on the show! I thought she was a great character and I liked her storyline with Callie. I thought they had chemistry. Lexie got on my nerves in season 4. I like her better later on in season 5. The relationship between Izzie and George was horrible! They totally destroyed their friendship for me, which was beautiful until then. The MerDer drama wasn't as bad as before, but the storyline with Rose really was not necessary. But I really loved what Meredith did in the last episode with the candles. That was one touching moment. Rebecca got on my nerves, but I liked how Izzie was a good friend to Alex throughout the "pregnancy"-storyline.
season 5: The first half I didn't really like, but it got better in the end. I love Cristina/Owen and Mark/Lexie. And I'm so glad Alex and Izzie are finally together, though the thing with Denny destroyed some of their moments for me... I love that Meredith and Derek are finally on stable ground in their relationship. I like them so much better when they are happy.
So, I can't really decide which season I like best because all of them had their good and bad moments. But I think if I had to chose, I'd chose season 5 because of the lack of MerDer drama and all the beautiful couples on the show!^^
Okay, I know I only commented on the personal storylines and not the medical cases, but I think their are good and bad ones in each season and ones that are more touching than others...
