I've watched this show ever since I caught the first season on ITV here in the UK. It's amazing, probably among my favourite TV shows, definitely my favourite TV show that's still going since The Shield, The Wire, The Sopranos, Lost and possibly 24 are the only others I like better.
In terms of how good the seasons are:
Season 4
Season 2
Season 1
Season 3
Season 5
I thought Season 5 was by far the weakest season, and I couldnt imagine many people thinking otherwise when I saw it. The only few other people I know who watch it also thought so. It wasn't particularly bad or anything, just average.
The best season is a bit trickier though. Season 4 was very well done and had a very good conclusion, so I might put that above season 2. The thing about season 2 is it has the best storyline in Dexter - the Bay Harbour Butcher and Doakes storyline - and one of the worst things in Dexter, Lila. She's easily the most annoying of each season's guest star. So it's hard for my to justify saying season 2 is my favourite when it includes her storyline.