Good thread.
I have watched most of the stuff people have posted on here already. Some of the series are very mainstream but still high quality such as 24, Prison Break etc.
The more interesting series are those which some of us might have missed due to being on obscure chanels or late night.
I know that I am repeating many of the earlier series people have mentioned, but I thought I would add some of my favourites and maybe try to include some that haven't been mentioned yet.
The Wire
NOT over rated at all. Believe the hype for quite possibly one of the best series ever on TV
The Shield
If you like The Wire then you will almost certainly like this too.
Cop show in an NYPD Blue style (be prepared for lots of shaky camera work). Michael Chiklis is superb in the lead role as Vik Macky, the leader of a small elite team called "The Strike Team" who use morally dubious methods (putting it lightly!) to combat LA's drug culture.
Complete with many shocks along the way and a final episode that does not disapoint with one of the best endings I can recall. Understated but apt.
Mad Men
Believe the hype again. If you like thoughtful clever stuff and can tollerate it's slower than average pace then this will reward you for checking it out.
Sons Of Anarchy
On a par with The Wire almost but has more charismatic characters. Shakespere on Harleys.
Breaking Bad
Geeky teacher makes meth and becomes embroiled in local drug industry. Fantastic series and can't wait for series 3 Yo!
Boardwalk Empire
Well worth a watch. Buscemi plays, well, Buscemi! A scouser plays Al Capone (very well!) but the best character is the main character Jimmy. Can't wait to see where series 2 goes.
Dexter
Darkly comic series of a serial killer blood splatter specialist working for Miami P.D.
Each season has it's own main story line with a different adversery for Dexter. Even if you don't like it, watch season 3 for John Lithgow acting everyone totally off the screen as a psyhco serial killer.
Battlestar Galactica
I concur with all the recomendations made by others. Excellent series which is so much more than sci-fi. Would be brilliant even if not set in space. Crocket and Tubbs boss is a decent actor!
24
Edge of the seat stuff in all 8 seasons. My better half just spent the last couple of months watching this on her ipod each night!
True Blood
Don't fall into the trap of one of my work colleagues who didnt watch this because he thought it would be like Twighlight or the vampire diaries!
A series which is very different to anything I've seen before, even if the opening credits and music are a carbon copy of Natural Born Killers!
Weeds
Single mom sells weed to pay the bills and feed her 2 kids. Fantastically funny in places with a gripping story all the way.
Entourage
Don't make the mistake I made thinking this is some kind of MTV style crap. Follows an actor from New York who makes it big in LA and his buddies who tag along for the ride. Typical HBO style program, lots of swearing and sex. Lots of famous cameos including James Cameron, a porn star, Mark Whalberg (exec producer of the series). Best charcter just edging a career defing role of Jeremy Piven as potty mouth agent Ari Gold, is Kevin Dillon as "Johnny Drama". They could quite possibly put a picture of Drama in the dictionary as the definition of the word
doofus!
Murder One
Can't recall off the top of my head exactly when this aired but I think it was around 94 or so? Stephen Bochco (NYPD Blue creator).
Sooo many modern series owe so much to this series. It was the first series (that I can recall) to be based around a single story line. Something which is now so common in TV that we take it for granted, before this each episode stood on it's own with little story line continuing from ep to ep. In this case a famous actor is accused of murder and this follows his trial. It introduced me to the work of Stanley Tucci who is excellent in this as is Daniel Benzali as the defence lawyer. It may have dated, I havent watched it since it aired in the mid 90's on BBC2.
Flight Of The Conchords
Kiwi duo in a band of the shows title. Fantastically original, clever and funny. Each show has at least one humourous song and music video which could cut it on MTV in their own right. With quotes such as
Mugger: What is this? Why does this phone have a camera glued to it?
Bret: It's a camera phone. Jemaine wanted one for his birthday.
it is well worth tracking down.
And now for some less heavy and generally less cool to admit you like stuff!
Hawaii 5 o
The remake staring Scott Caan and that other bloke with lots of others we've seen on other stuff!
Much more light entertainment than most stuff that people have listed. It may not be as cool to list this as the others, but I have thouroughly enjoyed this bit of escapism through it's first season which works so well mainly due to the buddy buddy relationship of the 2 main characters.
Modern Family
Whilst on the theme of main stream light entertainment I have to mention this 30 minute comedy.
The funniest new show for many years. Every character is superb, from Jay (Al from Married With Children basicly playing an even grumpier Al) to Cameron the over the top chubby gay guy to Phil the dad who thinks he is the coolest dad alive and never thinks he is wrong (mmm did they base this guy on me?!) to Manny the pre pubescent chubby Columbian kid who is 12 going on 50.
Each show has loads of laugh at loud moments and at least one proper bully busting moment.
30 Rock
An Emmy winning machine comedy with Tina Fey and Alec Baldwin. Solid comedy every show.
Peep Show
Brilliant Chanel 4 British comedy.
The Office US version
Once it found it's own identity this long running show is as good as the original if not better. Every office has a Dwight!
Eastbound and Down
Kenny *&%£$ing Powers! Danny McBride as a washed up US baseball star who as a sexist as it gets. Very funny comedy which showcases the best of McBride.
2 Season so far, 3rd to come soon.
CSI
By the book formulaic crime hijinks fun.
CSI Miami
As above but with added gratuitous slo mo sunglasses action.
CSI New York
As original, but with added NYC snow and Gary Sinise
Scrubs
Decent hopsital comedy complete with against type
My Name is Earl
White trash comedy with loads of brilliant characters, none better than Randy though.
Chuck
Nerd IT support guy gets super computer accidently put in his brain and becomes CIA agent. Very good brainless fun.
Terminator The Sarah Connor Chronicles
2 Seasons of suprisingly good action with solid story.
Episodes
Sort series 1 so far about a couple of British Comedy writers and their attempts to take their comedy show from the UK to the US. Co starring a thick cocked Matt Leblanc!
Things I am ashamed I have not watched but intend to watch asap.
The Sopranos
I know I will love it, just never got around to watching it.
Deadwood
The West Wing
Community
Good comedy where I have watched the first 5 or 6 episodes. 2 seasons to watch.
NCIS
House
7 whole series to watch!
Treme
Loved the wire, so should love this.
Arrested Development
Family Guy 
There are so many more I simply can't remember them all.
Hope this list points some of you in the direction of some quality stuff you haven't caught before.
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