Best:

1) Theon/Reek [Even with the lack of casting, I still think it'll be the best story arc. The fact that D&D have said many times that Theon is their favourite characters gives me hope that will for the most part do his ADWD story justice]

2) Arya [I love her character so much, and D&D have done an excellent job with her material so far [aside from whitewashing her in season 2, but they made up for that in season 3, so I can't stay to mad], and I expect no different in season 5]

3) Jon [Jon's story last season was arguably the best, and I think Jon will have another great season]

4) Cersei [Even with some cuts, I have faith that Cersei's story will be great]

Stories that will be good, at times maybe great:

1) Tyrion [I am very happy they are speeding his story up, as there is a lot of filler in book 5 regarding Tyrion, and the show may have some filler, but for the most part it'll probably be a little more interesting than it was in the book, and obvious sped up]

2) Dany [Dany's story last season was pretty good, sure Emila's acting was off in some scenes, but nothing terribly bad happened in her story, but then again nothing terribly exciting happened. It was merely just good, and for the first half of season 5 I expect that, and then episode 9 will come around and her story will be awesome]

Stories I'm unsure of:

1) Brienne [I don't know what they are doing with her this season, so I can't comment]

2) Sansa [See above]

Worst:

1) Dorne [I'm not happy with the lack of Arianne, and I'm also not happy with them having Jaime [who is my favourite character] go to Dorne when he should be doing other things like his story in AFFC. I expect great acting from Nicolaj, Siddig and the guy who plays Areo. Trystan and Myrcella are the scenes I'm dreading the most, because D&D do not have a good track record of creating love romances [ex. Robb and Talisa]]