Cards that combo with watchtower/watchtower counters:
disclaimer: many of these cards belong in multiple categories. There are also cards which you could make a case that they should be in this list. There are probably cards on this list that don't really combo with watchtower. In all cases, I made my best guess as to if they should be included/where they should be put. But, I'm not really all that good at dominion, so I am likely wrong several times.
Cards are in order of set. I may not have gone chronologically though, I just went through the card lists on the main page. I believe I looked at every single card.
Cards that combo because of draw-to-6
cellar
moneylender
festival
baron
minion
native village
fishing village
warehouse
Island
salvager
outpost
vault
forge
transmute
university
hamlet
horse traders
remake
oasis
noble brigand
inn
storeroom
mystic
ruined village (!)
plaza
black market
TOTAL CARDS: 26
Cards that combo because of on-trash ability:
remodel
embargo
trade route
mint
goons
hoard
cache
squire
market square
death cart
feodum
fortress
bandit camp
count
hunting grounds
doctor
masterpiece
merchant guild
TOTAL: 18
Cards that combo because of top-deck ability:
workshop
wishing well
ironworks
upgrade
smugglers
treasure map
expand
develop
trader
haggler
border village
farmland
hermit
urchin/mercenary
procession
rats
catacombs
graverobber
altar
stonemason
butcher
governor
TOTAL: 22
Cards that are countered by watchtower:
bureaucrat
militia
witch
swindler
torturer
ambassador
cutpurse
sea hag
ghost ship
mountebank
familiar
young witch
jester
ill-gotten gains
margrave
marauder
cultist
taxman
soothsayer
followers
TOTAL: 20
TOTAL OF TOTALS: 86
winner of the bit-of-everything contest: soothsayer. countered by watchtower, topdeck gold w/watchtower, trash curse w/watchtower, and decreases handsize for playing watchtower. I would not have included it for any one of those alone (trashing curse doesn't let you draw a card, topdecking gold isn't amazing, and decreasing handsize by 1 terminally isn't much either), but all together makes it easily worth of inclusion. Honorable mentions: Stonemason (not countered!), and Jester (never used for topdeck and trash in same action)