The RTS is a true workhorse. I've spoken to a few transit geeks that claim the RTS is the most rock-solid mass-transit bus ever produced. RTS busses were produced by General Motors from 1977 until 1987, when GM sold the design and patent to Transportation Manufacturing Company (TMC). Most of the Disney RTS busses were produced by TMC. The biggest problem with the RTS is that they are not a low-floor bus (translation: there are steps to get in), and the lift mechanism fitted onto the busses is kludgy at best.
The Nova LFS (Low Floor) busses are made by NovaBus, a Canadian company. In Disney's original testing, they fared ok, but apparently, some drivers complained about the reliability of the vehicles, leading Disney to pursue other alternatives. The Nova LFS is most easily identified by it's distinct single-pane arched front windshield.
The Gillig Advantage is made by the Gillig Corporation. Most easily identified by the fact that these busses have no rounded edges, even the wheel-wells are straight lines. These busses have reportedly received high marks from guests and drivers alike, and, at least based on my time spent on the WDW roads, are the most numerous in the fleet.
Interesting note about the displays on the busses. The display units are manufactured by Twinvision NA. One of the older versions of the software on the units used to be able to display mouse ears on the "Mickey Mouse". Unfortunately, this feature was removed in later versions of the software
(A lot of the information contained in this post came from posts by Tyler (Chip 'N Dale Express) on the Transportation board over the past few years)