What series is BMW known for the best?

I would say the m series just because when ever I hear someone mention a bmw they always mention a m3 or m5 or sumthin. I never hear someone say anything about the other series.

The 3ers are the bread and butter of BMW.

Definitely the 3-series. The 2002/3-series basically invented the affordable sports sedan segment of the market and remains its most successful model. It continues to amaze me that BMW can offer a more expensive car then most of the competition and still manage to control the market. It defies economics. The 3-series is to BMW what the Accord/Civic are to Honda and the Mustang/Taurus are to Ford. After entry level sedans (Civic, Corolla, Focus) and family cars (Fusion, Accord, Camry), the entry level sports sedan is one of the biggest segments.

Amongst car guys, the M models will be mentioned more then anything else, simply because they are the sportiest models. The percentage of M-car owners that race their car is much higher then regular models.

Just to give you an example, I just ran some numbers on a program I have access to through work...

In the Dallas/Ft. Worth area, from January 2007 to June of 2007, there were 2343 BMWs sold. 1784 of them were 3ers.