Best Brunch in Back Bay/South End
agreed with MC slim on all counts. 647 is probably my fave south end brunch-- they may not do them anymore, but the fresh donuts are outrageously good... they come in a paper bag filled with cinnamon sugar--the server shakes and dumps on a plate that has a ramekin the size of my head filled with sweet mascarpone, and a side of mulled cider. it is awesome. good bloody, too.
further afield, the blue room and east coast grill both bear the marks of having had chris schlesinger around at some point in their lives (and in the case of the grill, still). blue room has huge, killer buffet, and east coast grill has huge, killer BLOODY MARY buffet. yum.
A much-discussed topic here: searching will yield many other opinions.
The South End is a superior brunch neighborhood. The locals pack places, and the restaurant density makes it practical to walk around and look for friendly menus and wait times. Many have patio seating and excellent eye-openers. My favorites include Union, Caffe Umbra, Metropolis, Tremont 647, Stella, Aquitaine, 28 Degrees, and Mike's City Diner (more a traditional breakfast than brunch venue). Toro and Orinoco are tops on my to-try list.
The Back Bay is far less interesting. Sonsie, Laurel, and the Trident Bookstore & Cafe are decent and reasonable; I especially like Sonsie for its Newbury Street facing cafe seating. The Bristol Lounge is nice, but pricey, as are a few other hotel restaurants: Brasserie Jo, the Cafe at the (old) Ritz, Azure, Aujourd'hui, the Stanhope Grill/Cuffs (nice patio). You can do Tapeo starting at noon: decent tapas, lovely patio and drinks.
Further afield, my favorites include: East Coast Grill (Latin flavored), Centre St Cafe (veggie-friendly), Doyle's (great old-bar atmosphere), Deluxe Town Diner (great griddle cakes), The B-Side (serves late, solid combo of lunch and brunch stuff, and great eye-openers), Blue Room (huge, fancy buffet), Sel de la Terre (elegant, sunny, great lunch/brunch menu), and the Beacon Hill Bistro (breakfast, followed by brunch service).