S2 has the best structured plot. S1 hardly has a plot. S3 and 4 meander about. S3, especially, uses a lot of macguffins. S5 and 6 are on a level with S2. S7 really wants to have a plot, but it can't quite get there.
Take "Passion" for example. The episode really needs to be there. The stakes are Angel's soul and Jenny's life. These are things that the audience care about. "Choices" from S3 is also a great episode, but it is centred around a box that we've never heard about before.
As a rule of thumb, if it is easy to remember the order of episodes in a season, it usually means that the season has a nice linear plot, where each episode moves the plot forward. I find the timeline of S2 very easy to visualise. S4 is great season, but it is much harder to remember when the important beats occur.