My initial thought is that it would be somewhere in the U.S. because the U.S. has some brands that aren't overseas (SpringHill Suites and TownePlace Suites?) and I don't think there are many (any?) international only Marriott brands (except maybe Protea in Africa?). Las Vegas is the home of hotels but I'd guess they're much more casino brands, my next guess would be New York. I count 227 hotels in New York made up of 26 brands:

Edition x 2
Ritz Carlton x 2 (one of them is a "NoMad" but I don't think that is a separate brand)
Luxury Collection x 1
St. Regis x 1
W x 3
JW Marriott x 1
Marriott x 17
Sheraton x 9
Delta Hotels x 4
Westin x 6
Le Méridien x 2
Renaissance x 7
Autograph Collection x 5
Tribute Portfolio x 1
Design Hotels™ x 2
Courtyard x 47
Four Points by Sheraton x 5
SpringHill Suites x 15
Fairfield x 29
AC Hotels x 3
Aloft x 7
Moxy x 6
citizenM x 2
Residence Inn x 36
TownePlace Suites x 10
Element x 4

I guess a more interesting question is, where is the most cost effective place to visit to get all brands? I'd guess Kuala Lumpur is definitely up there, it is very cheap here. New York is probably the most expensive.