I started watching Southland in season 4, and liked it so much that I decided to start watching from the beginning. Season 1 is an excellent season of TV that throws you right into the world of the LAPD without any hand-holding. The cast is large, as the show covers the patrol officers and detectives on their day-to-day lives on the job. The pace is frenetic and the acting is so raw and realistic that the show at times feels more like a documentary than a scripted drama. The acting is excellent, the cases usually very strong, and there is a clear arc over the course of the season.
If you are new to Southland, season 1 is as good a place as any to start. If you came to the show late, like me, it is quite interesting to compare the characters in seasons 4 and 5 to where they started out in season 1, as certain characters have changed dramatically.
Is Southland the best cop show? As someone who isn't generally a fan of the drama, though I enjoyed NYPD Blue and early Law and Order, and consider The Wire one of the best dramas of all time, yes, I think Southland is the best cop drama. For me, it comes down to the characters, who are so unique and compelling and different from what I've come to expect of TV police.