2009, because that's what I have.
In all seriousness it depends on what your looking for and price range. This is a terrific platform based on the excellent e46 so it is pretty robust. There are quite a few of us with high milage that have had minimal issues (mine is at 103k). This is by far the best car I ever have owned (I bought new).
Now here is the breakdown:
2007 to 2010 - $15k to $25k
Lci refresh was in 2007 with new n52 engine and 6 speed auto. Avoid 2007 and early 2008 as they had major tranny problems. This was fixed with an updated tranny module in later builds (mine has this) and it works great. Still HIGHLY recommend reading the auto trans sticky and knowing the issue if auto transmissions are your bag. That way you can take a long test drive knowing what to look for.
If you like 6 speed mt all years are golden.
I personally like the lci models best as most all have color matched bumpers and new led taillights.
2004-2006 -- $9k to $14k
Older m54 engine but you still have an oil dipstick. Excellent engine but has some common age related failure points that need to be monitored at this point in their life (ie cooling system, DISA, ccv).
Main complaint for 04 to 06 (especially 04) is rough suspension and ride. If you ask UncleJ he will say it is very true for the 06 as well. Best to take a long test drive on rough roads to make sure you like it. In addition there were a few things left out of the early 2004 (ie Bluetooth )
Also panorama sunroof is a common complaint on all years...especially pre lci. I don't hear much about this in newer models (I haven't had issues), but best to use sparingly.
Last edited by x3brian; 11-28-2012 at 10:32 PM.