Just got back from a wonderful (early season) week on Nantucket and wanted to throw in my two cents. We were traveling with three families including 4 children between ages of 1 and 12 which somewhat limited our dining choices.
Company of the Couldron: Recently voted most romantic restaurant on Nantucket, and for personal historical reasons a must visit for my wife and I. This little (38 seat) restaurant has a set menu for each night that is posted on the web and at the restaurant. They are willing to make minor substitutions, but for the most part you get what they are serving. We will generally check out the menu early in the week and decide which menu looks best and plan the rest of our eating around that date. Because of scheduling difficulties, we actually ended up going for the least appealing menu, but it was absolutely fantastic nonetheless. Because the menu changes daily, I will not bore you with a detailed description, but it started with a crab cake in a mustard sauce with a tropical salsa, followed by a simple salad of local bib lettuce with a marinated cucumber and black sesame topping (for such a simple concept it was unbelievably good) and the main course was a baked hallibut in a mushroom/lemon grass/spiced soupy sauce that made me want to lick the plate clean. The key lime mouse dessert was incredibly tart and delicious with creme anglais on the side.
Le Languedoc: A consensus choice by the five adults. Another fantastic meal with great service. The dining rooms are in a converted residence so each room only has a few tables, keeping the noise level down. Interesting menu with well executed food. By far the best was the grilled lobster with polenta stuffing that my wife got - tender, succelent lobster with creamy, flavorful polenta. The lobster meat was easy to extract from the tail and the claw meat was already removed. I had the lamb which was tender but unexceptional. My brother in law had pork cheeks - they were almost beef-like in flavor and consistency. Excellent wine list, good desserts.
American Seasons: My sistera nd her husband went the night I was at Company of the Couldron. They were extremely satisfied with their meal. My sister couldn't stop talkng about the Foie Gras Creme Brule and both said the duck roasted with cherries was the best duck they had ever had.
The West End (Madaket)- Avoid this place like the plague. The food was horrid. The service abominable. The bathrooms filthy (I didn't notice, but then, I am a guy). This is your only choice in Madaket (the best place to watch the sunset) but don't choose it. Eat in town and drive out for the sunset. Or pack a meal. Or scrounge in the trash bins at the beach. Just don't go to this restaurant.
Provisions: A sandwich shop on straight wharf that makes great sandwiches (disclosure: I worked there 10 years ago when I lived on the island for a summer.) I recommend stopping for sandwiches before getting on the ferry to go home. The veggie sandwich is wonderful - full of sprouts, carrots, artichoke hearts, etc with a somewhat bland hummous.
Taco Taco: A little tex-mex dive on stemship wharf that makes fast, wonderful tacos, taco salads and burritos. The 8 and 12 year olds would have eaten every meal every day at Taco Taco. I would avoid it on Monday because they tend to run out of about half the ingredients and don't get restocked until late on Monday afternoon.
Ropewalk: New jamaican/carribbean inspired menu with some interesting choices. we stopped in for lunch one day. The ceasar salad was crisp and spicy, the tuna fresh and well cooked, I wasn't hungry that day and just ordered a beer, so I cannot provide a detailed food report.
There were a lot of other restaurants with very interesting menus that I wanted to get to, but couldn't. Oran Mor, the Pearl and 21 federal all looked wonderful.