"The Best"-- within walking distance of Marriott Downtown Waterfront??
My advise would be to ask the front desk at the hotel for a map of the street car and MAX routes in downtown (both lines are free), then walk across Front Avenue to the Full Sail tap room for a beer and ask for some dining recommendations. From there walk another block south and catch the street car to the rest of downtown, including into the Pearl District as well as to connections to the MAX light rail. You can cover a lot of area with very little walking.
I second grapesgrapes suggestions. Yes, other suggestions in this thread are closer, but none are "must eat"s or "the best" as you specified, except Clyde and maybe Higgins. OP, Gruner, K&Z and Bunk are all perfectly walkable fro the Marriot, especially now it's warming up. How lazy are you people?
If you are from a place without good sushi, Murata would be my closest favorite choice in that area. If you are willing to walk a little bit, perhaps Clyde Common. MichaelG's recs below are other choices close-by.
If I had to have a lunch I could walk to near there: Hot Pot City or Tom's First Ave. Bento
1014 SW Stark St, Portland, OR 97205
Hot Pot City
1975 SW 1st Ave Ste J, Portland, OR 97201
Not too far is Higgin's - my favorite in this area. 7 blocks back from the Marriott and 1 block over. Always excellent food, great wines and beers, and a focus on locally sourced ingredients. Some of the preparations are really unique and get the taste buds firing on all cylinders.
1239 SW Broadway, Portland, OR 97205
These all seem like pretty odd suggestions in terms of walkability. Great places all, but none are close to the Marriott!
In terms of proximity, I'd suggest Veritable Quandry - nice bar, good food, Carafe for classic French, or Nel Centro - also a good bar, and Mediterranean food. if you like food carts, check 'em out at 4th and College. Thai Pasta is one of the best there, and I'm addicted to the tom yum fired rice.
1408 SW 6th Ave, Portland, OR 97201
The closest "musts" to there for me are Grüner (one mile), Kenny & Zukes (also one mile), and Olympic Provisions (1.1 Miles). Grüner is amazing "alpine" food, K&Z is an upscale deli with amazing pastrami, and Olympic Provisions is kind of an artisinal salami shop that also does very interesting sit-down dining. If you are interested in something that's a *lot* more casual, the sandwiches at Bunk and Bunk Bar receive national attention; Bunk Bar is a 0.8 mile walk from your hotel.
Closer in than that, there's nothing I'd consider special. If you end up needing a reliable meal right near the hotel, Veritable Quandry is fine.
Kenny & Zuke's Deli
1038 SW Stark St, Portland, OR 97205
107 SE Washington St, Portland, OR 97214