It is a fair question, and it gets asked because the photographs are so consistent that they start to look like marketing. The honest answer is that Thailand is beautiful in several unrelated ways, and which one you meet depends entirely on where you go. The flat central plain, the forested north and the limestone coast of the Andaman have almost nothing visually in common.
The ten places below are chosen to cover that range rather than to rank anything. Several can be reached from Bangkok in a morning; two of them are a flight away and worth the flight.
Ten places to see
The Grand Palace and Wat Phra Kaew, Bangkok
The walled royal compound on the river holds the most ornate temple in the country, and going early is not optional advice so much as the only way to see it properly. Shoulders and knees covered, and expect to keep moving with the crowd rather than lingering.
Wat Pho and Wat Arun
Two temples within a short walk and a river crossing of each other. Wat Pho holds the reclining Buddha and a long courtyard of stupas; Wat Arun sits on the opposite bank with a steep porcelain-studded tower you can climb partway up. Late afternoon light is kinder to both.
The floating markets west of Bangkok
The canal markets in Ratchaburi and Samut Songkhram are busier and more photographed than they once were, but the surrounding waterways are still working canals. Go at dawn if you can, and take the smaller of the two markets rather than the famous one.
Ayutthaya
The former capital an hour north of Bangkok by train, now a spread of brick temple ruins across an island between rivers. Hire a bicycle at the station and ride between them; the scale only registers when you cross the distances yourself.
Sukhothai
Further north and quieter, the older capital sits inside a landscaped historical park of ponds, walls and seated Buddha figures. It is more composed than Ayutthaya and far less visited, which for a solo traveller is much of the appeal.
The old city of Chiang Mai
A square moat around a grid of lanes, wooden temples and small courtyards, with forested hills rising immediately behind the city. It is the easiest large Thai city to walk, and the temperature in the cool season makes walking a pleasure rather than an endurance test.
Koh Chang
A large mountainous island in the east, close enough to Bangkok for a long weekend by bus and ferry. The interior is rainforest and waterfalls, the west coast is a run of beaches, and the island is big enough that you can find a quiet stretch even in the dry season.
Krabi and Railay
The limestone country of the Andaman coast, where cliffs drop straight into the sea and some beaches are reachable only by boat. Railay is a peninsula rather than an island and feels cut off in the best way; the walk over the headland to the lagoon is short, steep and muddy.
Khao Sok National Park
Old rainforest and a reservoir full of drowned karst towers in the southern interior. The lake is the photograph everyone has seen; the forest walks behind it are the reason to stay a second night.
Erawan Falls, Kanchanaburi
A seven-tier waterfall in the west, each level a separate pool in the forest. The climb to the upper tiers thins the crowds considerably. Kanchanaburi itself, on the river, is an easy and unhurried base for a couple of nights.
Thailand does not have one landscape, so it does not have one answer. Pick two of the ten that look nothing alike and you will see why the question keeps coming up.