Def one of the most covered topics..
You can go with H&R Sport Springs on stock shocks/struts (I went this route and am very happy); you could couple the H&R Sport Springs with Bilstein shocks or you could go with H&R or KW V1 / V3 coilovers
There is also the Dinan suspension upgrades (stages 1,2,3) for the 335xi
Current: 2015 Audi A6 3.0T
Gone but never forgotten: 2009 E90 LCI M Sport