Best at what? How much are you willing to change? On what budget?
With sway bars, bigger isn't always better. The stiffer the sway bar, the less "independent" an independent suspension becomes (ie, it starts to act more and more like a solid axle). You can also make handling worse by combining too much bar with too little tire. Sway bars aren't meant to be the primary method of controlling body roll, that's best done with springs and to some degree the dampers. Sway bars should be used as the final level of fine tuning a suspension's balance. Springs, dampers, roll bars, and tires must all be selected carefully in order for them to work together properly.
04 IR 330i ZHP | ESS TS2+ SC | MagnaFlow "headers" | KMS 3.64 LSD | AST 4100 550/650| APEX ARC-8 18x9 | H&R front sway bar | Vorshlag camber plates, RTAB limiters | TMS subframe & swaybar reinforcements | UUC SSK & DSSR | UUC TM & TME | BW motor mounts | Dinan strut & shock tower bars | vaio 4-bolt oil pump