My wife and i will be staying at the Beacon hotel on the upper westside. Can anyone recommend a good place for Sunday brunch.

Do we assume you're referring to New York City?

Sorry folks, that is New York city.

Sarah Beth's on Amsterdam and 81st, but expect long lines unless you get there early, certainly before 9:30 - 10:00

Sarabeth's is a classic, and yes there will be a line (no reservations taken).
A more casual place a block or so up is E.J.'s, a retro diner with much better than average diner food, also with long lines. Actually, anywhere on Amsterdam avenue (one avenue over from roadway)between 79th and 87th is a good place to look for brunch places, including some ethnic choices like Cafe Con Leche , Sabor, Barney Greengrass.
Keep in mind that brunch in NY is later than lunch (busiest at 1-2 PM ) rather than a breakfast/lunch combo. You'll still find breakfast dishes on the menus at most places, but if you're planning on going before noon, lines will be much shorter almost everywhere.

Skip Sarabeth's and EJs and go to Nick's Meditteranean on Broadway and 76th (75th??) It's amazing and there is never ever any line. They have a huge menu, which can be daunting, but you can't go wrong with the omelettes, or fresh fish dishes. They're great for dinner, too. I plug it so highly because my husband and I live on the upper west and can't get over why the hell people wait in line in NYC for mediocre brunches! Also, as someone mentioned, if you can go before the crowds (before 11:30) you're better off wherever you go. Also good is a place called "Good Enough to Eat" on Amsterdam b/w 83-84. Great food and great atmosphere,,, with a line.

Of course, brunch at the Plaza can't be beat. Then you can walk off some of it through Central Park.

The Cotton Club has a nice Sunday gospel brunch buffet. $20 all you can eat and nice music too.