Which TV series is the best one for studying English?

There are many good TV series, but they are all different. If your level is not very high - then it is a good idea to start with short ones as to the length of the episodes, like:
1. How I met your mother (25 mins)
2. Everybody hates Chris (25 mins)
3. Friends (25 mins)
4. Sex and the City (25 mins)

Their second advantage is that they are not biased, have no special terminology as medical ones, for example:
5. Scrubs (25 mins)
6. House M.D. (45 mins)
7. Grey's Anatomy (45 mins)
or scientific ones:
8. Big Bang Theory (25 mins)

If you feel confident you can watch longer series, some of them are already listed +
9. Desperate Housewives (great grammar!!!)
10. Devious maids (good grammar)
11. Go on
12. Gossip Girl
13. Sherlock
There's also "Extr@ English" - an educational series for lower levels!

Watch them with subtitles and then without, it depends on your level, anyway usually at first it is difficult, but it takes time and patience, so STAY STRONG AND GOOD LUCK!