Probably my favorite is the early morning sunrise from almost any hotel balcony where I can see greenery. Bonus if there's a lake/pool in view, and not much else. Very late at night is also fun.
MK - can't really think of any I visit very often, except maybe people mover. For quiet, we head to a monorail resort via small shuttle boat.
Epcot - sometimes rose garden (not so much to sit), Imagination fountains, quiet areas tucked around WS
HS - walkways outside the front gate, small area by RnRC can sometimes be quiet
AK - Oasis, sometimes parts of Pandora at night
Other -balcony at CR/WL/AKL, All CL lounges at 6-7am, fountain courtyard at Royal Pacific, quiet pool areas at Portofino/CR/BWK/Dolphin, Epcot hotel area paths late at night/early AM, almost any hotel courtyard late at night/early am, there a pretty fountain at GF- towards the new DVC
Other other - TPA has some nice semi-quiet spots and very pretty art, JW Marriott has lots of quiet areas along the lazy river