I go back and fourth between the 4 adult seasons.
S8 was good because it was very Brooke-heavy so I loved everything tied to her (the adoption, the babies, all the stuff leading up the wedding, the hurricane episode). But I didnt like how Haley didnt have much to do (understandably, since Joy was pregnant, but still), Nathan's SL with the teacher kind of bored me, I hated how rushed they made Brooke finally have a baby/her pregnancy, how much it focused on Chuck and other stupid things.
The beginning half of s5 was great, even though it was very LP-heavy but the last half bored me.
S7 I thought was great all the way around, and there very few filler eps. What I esp. love about it is that I felt like they acted younger during that season which was much needed. Love the ep where the boys go camping and the 3 girls stay back and eat pot brownies, the 80s episode at the school, I loved Alex's character during that season, loved the Lydia storyline and other things. A drawback would be the Grubbs/Miranda crap and the Julian/Brooke stuff annoyed me too because I felt like it wasted time. If Brooke was really that untrusting of Julian, she wouldve been so during that year flashforward from s6 to s7 and then we wouldnt have even needed to see it. They had been together for a year and a half- Brooke even wanted to get married for goodness sakes- yet we are supposed to believe that Alex was some major threat to them, even though Julian was surrounded by actresses in LA during that year?? Oookay. So I feel like if they took all of that out, they couldve had Brooke plan her wedding that season and do the baby stuff in s8. Also it was pretty Chuck-heavy too.
S6 was great because I loved Brookes robbery SL, Qs SL, Sam was pretty good, the movie SL, etc. There were a lot of filler things though that I didnt care much about, like the USO show one and other things. S6 was probably one of the only seasons where I enjoyed watching LP stuff though (like the one where Peyton moves in and the one where they are babysitting Jamie and Andre) so thats a plus.
So essentially each season has its good and bad so its hard to list them. I guess if I had to, Id say s5-s7-s6-s8.