I don't know what "the best" is, but wireless speakers comes in three different types......
IR (Infra-red) - uses an infra-red signal (much like a remote control).....does not need line of site, as the IR signal can bounce off walls (i.e. point your VCR remote at the wall behind you and the signal will bounce forward and operate your VCR
Radio frequency - There are two different ones which will come in either 900 mHz (same frequency to most cordless phones) or 2.4 GHz (also used in cordless phones such as Siemens, VTech). The downside with 2.4 GHz is that it operates at the same frequency as wireless networks and your microwave oven. So neither the transmitter, nor the receiver, can be positioned within 8 feet of one of these, or else the signal will pop, hiss, and generally sound like crap. The 900 mHz may also experience interference, or cause interference with, your 900 mHz cordless phone.