It depends on what you want from the thermometer.
If you want remote temps, then the Maverick ET372 w/ dual probes (one for the meat, one for the cooking chamber, aka "the pit") seems to be a popular choice.
If you are thinking about getting an automatic draft controller for the WSM, both the Stoker Controller and the DigiQ units have multiple thermometers built in, as they need to monitor both pit temp and meat temp to be able to control the blower properly. Some units have remote reading capability via ethernet / wireless network.
If your lid gauge is calibrated, and once you get to know your smoker, you can do a lot with just a digital pen thermometer. The Thermopens are excellent, but pricey.
