I've taken my folders extensively on buses:
-For under bus storage any folder will do. Although the coach type long range commuter buses typically have these compartments underneath the bus, drivers do not usually allow passengers to use it because it takes too much time for the driver to open/close the compartment. The first time you ask the driver make sure you bag the bike; If he says yes you probably won't need to bag it in the future, but if he says no you're SOL. Also beware that you might not get the same exact bus or same driver every day and, thus, might not have access to the storage you were counting on.
-The overhead storage also available on coach commuter buses is extremely narrow as are the isles. You will not be able to fit any bike there except a Carryme or, possibly, a Strida. You will not fit a Brompton, a Tikit, or a Dahon. It's also a hassle to get luggage onto these overhead racks...especially when you're trying to get to in/out of your seat in a timely fashion; The other passengers will become irritated rather quickly if you delay the bus or if you accidentally drop your bicycle on them while rushing to get off at your stop.
-Anything which fits in the overhead storage space on a coach commuter bus will also fit in between your legs...perhaps not comfortably, but the minor discomfort is well worth the ease of getting on/off the bus.
-Short range "city" buses have more floor space, but since they're also typically more crowded with more people getting on and off the situation isn't much easier: Unlike the coach commuter buses here you might be able to manage with a small 16" folder like a Brompton or Tikit
if the bus isn't too crowded. However, since these buses are slower and usually don't travel more than 10 miles, in these situations it's usually more expedient to just ride your bike.
Forget about 20" wheeled bikes and forget about 16" wheeled bikes which are barely even 2" smaller.
This is the only practical option:
P.S. A small towel/rag to wipe down your bike is much more useful than a bag on rainy days.