From the action packed season opener, to the scream at the tv 'to be continued', at the end of the finale, it's very obvious this is Smallville's best year.
Season five takes place straight after the season four finale, with an episode entitled arrival. It ties up many loose ends, and resolves many issues raised in season four. Most notably Clark finally finding out that his best friend, Chloe Sullivan knows his secret.
Throughout the season there are many ongoing story lines. The on again, off again Clana relationship, which for me became old in season 1. I'm a little bias, but it seems to me that Chloe and Clark should be the ones together. Fortunately we are treated to many Chlark moments this season. Due mostly to the fact that Chloe has now become Clark's confidant, and keeper of his secret.
Then there's Lex's progression to the dark side, where he becomes ever closer to his inevitable destiny. Developing into a character, who is more Clark's enemy, than his friend.
One of the most interesting, and gripping plots however remains the Professor Fine AKA Brainiac storyline. Brainiac, a Kryptonian artificial intelligence wants nothing more than to release ZOD. And this means a lot of trouble is brought Clark's way. Leading to an inevitable clash in the finale, which leaves many characters fates hanging in the balance.
This is Smallville at it's best, if you do only buy one season of Smallville then this should be it!