The story for 4 was great. It's the only aspect of 4 that gets praise (although I think the graphics are underrated AF, those cutscenes on 360 are unreal).

5 had a shit story, but it had phenomenal world building. Evil Cortana was actually a really cool villain (if you can get over them semi-resurrecting Cortana) and the Guardians should have been the new Halo. Also, the combat was awesome and they fixed the Prometheans to actually be fun to fight against.

BTW, I have a special affinity for the stand-alone stories. CE, ODST, and Reach are my top 3 in the series.