S2 and S5 are considered the "black sheep" seasons, but that's only by the lofty standards of seasons 1, 3 and 4. Season 2 starts a bit slow and introduces you to a lot of new characters, but towards the end it is on par with any other season in terms of building suspense and making you care about the characters. It's a lot better than it gets credit for.
People differ on the best season but the consensus is usually either 1 or 3, then 4, 5, and 2 in that order. I think season 3 is the best but all are better than 99% of all other TV shows.