The Tempe Transportation Center is safe and has a lot of RPTA-operated buses running through. You’ll catch an El Dorado 30’ there, but most other buses will be a variety of 2008 - 2013 C40LFR and the occasional Gillig. There is generally a lot of transit activity in this area compared to other parts of the valley.
Nearby you’re almost guaranteed to catch an L40LFR on the “FLASH” route. There’s a bus every 5 minutes from Packard Drive, about 5 minute walk from the transit center in Tempe. You can ride around for the whole circular trip (about 30 mins).
The Scottsdale 35’ Gilligs usually appear on the Miller Rd route or Neighborhood route. The east/west routes nearby in south Scottsdale (17, 29, 41) are all candidates for the XD60, D40LF, 2007 D60LF (Caterpillar powered), 5600 series gilligs and Phoenix-owned CNG gilligs. A little further north on route 50, you’ll probably see XN60, CNG Gilligs, D40LF or 2008 D60LF (Cummins powered).
DE60LFR are often seen on 77 Baseline Rd, which is several miles south of the Tempe transit center.
D40LFA and D60LFA often operate on routes 40 and 112 in the east valley.
Your best bet for a RAPID bus is in downtown Phoenix (45C-LFW and Xcelsior). If you go in afternoon peak, you can ride between the State Capitol and central station. Note that the I-17 route runs counter to the other rapid routes through downtown, so you can hop on that at central station, get off at the capitol, walk down to Jefferson St. and catch whatever shows up on the I-10E, I-10W, or SR-51 Rapid to go back to Central Station.
RPTA XN60’s are somewhat scarce.. you’ll see them in downtown Phoenix on the 533/535 but you can also sometimes catch them on routes 45 or 61.
D40LFR are also hard to come by as they mostly operate on the west-valley express routes. You’ll also see them in downtown Phoenix.