We were on Maui last March and enjoyed many delicious meals. The whole trip was great. We'll definitely go back, in no small part because of the food. Anyway, here's where we ate that I would recommend (it's a long list):
The Banyan Tree. This is the main restaurant at the Ritz Carlton. It's very expensive, as are most of the nice restaurants on Maui, but the food was terrific. The Ritz is in Kapalua, about 20 minutes north of Lahaina. The food was fantastic. There's a lot of fresh seafood with asian influences to the preparation. My main course was grilled tuna with seared foie gras on top. It was unbelievable. The tuna was quite lean and the super fatty foie gras complemented it perfectly.
David Paul's Lahaina Grill. Very good, as already mentioned. One tip: we had a hard time getting a reservation there but had no trouble finding two seats at the bar at about 8 o'clock.
Gerard's. This is in Lahaina, part of The Plantation Inn, a couple of blocks off of Front Street. Classic french cuisine, with a bit more seafood than in mainland french restaurants (naturally). Absolutely perfect service. We ate on the veranda.
Mala. This on Front Street in Lahaina, but a bit north of the main drag. It's small plate food, but done in some interesting ways. Casual. Beautiful setting right on the water. Lots of good beer on tap (including a couple of local brews).
If you are starving when you get off the plane because American Airlines ran out of food on the flight (!) and you really need to eat something right away (like we were), I recommend this sandwich shop called Ba-Le in the Maui Marketplace Mall near the airport. The pork sandwich there was great. It might have saved my life.
There's a poolside restaurant called Maui Onion in a hotel (I forget which one) in Wailea. The onion rings there were unbelievable. They use local maui onions, which are kind of sweet. (The burger was just okay.) We had a tasty and fatty lunch there and then went down the road to Makena Beach for the afternoon.
A good casual option is Penne Pasta Cafe in Lahaina. The food is pretty good and the prices are low.
One place we tried but didn't like: Longhi's in Lahaina. The food was mediocre at best and was very overpriced.
Have a great time in Hawaii.
-Dan