Buses - the intra-city types:
or coaches - Greyhound / National Express / general Inter-City)?
Its never really occured to me to ask this before and I don't believe I've ever seen a topic on it. I ask because GeekToo is currently drawing a RV in the "Blender - new graphics" thread seems to have gone with the assumption they're coaches (not saying he's wrong).
The current buses in OTTD could be either, but the new pics will actually allow the player to see the difference.
Admittedly the Foster MkII "Superbus" does look like a Coach (which is the one GeekToo is modelling), but the Hereford and Foster are both more generic and personally strike me more as Buses. The MPS seems to predate the difference.