USB will give you the highest theoretical speeds, but WiFi is more convenient with the multiple device connectivity. If you tether on anything faster then HSPA(standard 3G) bluetooth 2.1 will bottleneck you. On anything higher then HSPA+ bluetooth 4.0 will bottleneck you.
USB 2.0 = 480Mbps - Best speed. Only one device at a time.
Wifi 8.02.11N = 300Mbps -Ideal for tethering. No bottlenecking on any network.
Bluetooth 4 = 24Mbps - Usable and minimal bottlenecking.
Bluetooth 2.1 = 3mbps - Not ideal for tethering. Not recommend.