Depends a lot on your budget. Thailand has lots of things to do from trekking, nightlife, tours, national parks and loads of beaches. Lots of historic places to visit. Roads are good, local and long distance transportation is good. Good hotels in all price ranges.
(Bangkok has excellent city transportation, cheap meter taxis when you use the meter, excellent city rail service too!)
http://www.urbanrail.net/as/bang/bangkok.htm
Many tourists make their trips a "twofer" where they take a side trip to another country for 4 or 5 days. You could do that from Thailand. Fly Round Trip to Bangkok. (Round trip from many countries is cheaper than buying 2 One Way flights.) During your stay in Thailand take a side trip to a second country. Depends on your income - fly round trip or go overland by bus or private transportation. Return to Bangkok for a few days for your flight back home. Thousands of tourists do this every year.
Cambodia and Laos is easy to reach from Thailand. Even overland. Your passport determines if you need a visa for Thailand or not. From the US, most of Europe, Australia, etc. no prior visa for Thailand. For most Westerners you can enter Thailand with just a passport and get a free Visa Exemption stamp good for 30 days. For Laos and Cambodia you will have to pay for a visa.
http://thaiembassy.ca/en/visiting-thailand/visas/tourist-visa-exemption
Some tourists with more available money even take a discounted flight to Singapore or even Hong Kong for a few days. There are many airlines flying from Bangkok. Bangkok has 2 airports, Don Muang also is open for flights. Many discount airlines use Don Muang.
For Thailand, hotels are more expensive during the Christmas-New Years holidays. Also, from many countries flights are higher priced than other times of the years. Some hotels can be fully booked during this "high season" period. This is when the weather is best. All of Thailand has summertime type weather. Some times of the year more heat or more rain. But tourists by the millions still show up every month of the year but more when it is cold in N. America and Northern Europe. Even during the rainy season you will find days with little or no rain. Anyway usually enough "sun time" to fry on a beach.
If you have never been to Asia before and you still intend to only visit one country then make it Thailand this time. Thailand is an easy "intro to Asia" country to visit.
Up to you.