Good hotel in Bangkok?

I would like a hotel that has a large complimentary breakfast buffet, is modern, and can accomodate 2 adults (40 yrs), a boy of 18yrs, a girl of 17yrs, and a boy of 15 yrs. A fitness center would be nice.
We're also looking to go towards the end of december and its very difficult to get availability. Our budget is less than 175 USD a night.

Try Pathumwan Princess located in one of the large shopping malls in the city. Be early coz the place is popular due to location. Modern, clean, pleasant and safe- definitely 4 star. You will also love the sumptious buffet breakfast spread.

It will be high season so try to book early. There are a lot of hotels in Bangkok so there's no problem getting a room even in high season.
In the $175 range I would reccommend the Sofitel Silom. It's a 5 minute walk to the night market and close to the Skytrain.
The breakfast is very good especially if you book deluxe rooms with a smaller private dining room on one of the top floors. There is a nice pool and gym.
If you can afford it, one of the nicest hotels in Bangkok is The Sukhothai. The breakfast, especially on Sundays is, is incredible. They have a great pool, gym, and tennis court.
Excellent hotel, Excellent service. Not as "nose in the air" as the Oriental.

My offer would be the Grand Mecure Frotune. A 4 star Hotel with rooms at about $80. In a new part of Bangkok.
1 Rachadaphisek rd, Fortune Town
Bangkok 10400 Thailand
+66 2 641-1500

Its a great hotel connected to a shopping center with lots of electronics and a TescoLotus. There are spas barber shops and jewelery stores over 500 shops.

Near:
Ratchada Night Bazzar, Lat Phrao intersection, is a new night bazaar similar to Silom's better-known Lumpini Night Bazaar, with open air shopping, eating and drinking.

Transportation
Phra Ram 9 Subway station Station is right near by and because of the Tesco/Fortunetown there are plenty of taxis always available.

I'd strongly recommend Grand Millennium Sukhumvit Hotel. Breakfast buffet spread was great and had a good assortment.The hotel is near lots of amenities, shops, pubs, bars, restaurants so walking is an easy way of visiting the area. Sukhumvit is lively but not noisy. It's also a five minute walk to the sky train so seeing the city as a whole is cheap and convenient. Also very friendly and motivated staff. I like the bath room very much. We can enjoy watching TV and movie from bath tub over the bath room window. There is even an audio speaker as well.
You can also refer map for its location http://www.discovery-thailand.com/Hotels%20Map%20Pages/Sukhumvit%20Hotels%20Map.html . The deluxe room is USD154/night (discounted from USD267) on this page. Have a nice trip!!