I'm going to suggest The Front Room. The last several times I have been there, the service has been much improved - in fact, quite good. (Same servers, hope they stick around!) I really, really like the eggs benedict and blueberry pancakes. Go early for the least hassle if hitting it on the weekend.
The usuals; if you Google, you'll find the menus for most, if not all, online.
• Becky's--nothing fancy, just good diner food at a good price, has received national attention, so often very crowded;
• Porthole, gussied up former dive, great location, pricier than Becky's, you can eat on the wharf if the weather's decent;
• Bintliff's, humongous breakfast/brunch menu, some very creative choices;
• Good Egg, shares space with PepperClub, good breakfast fare;
• Miss Portland Diner, diner fair with flair, excellent service, very good food (and free parking in the adjacent lot, always a plus).