One long west coast, six beaches, and a steady drop in noise as you drive south. Pick the point on that line that suits you.
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Almost everything on Koh Lanta sits along one road down the west coast. Saladan, the little town at the northern tip, is where the ferries dock and where the banks, dive shops and supermarkets are. From there the road runs south, and every kilometre takes away a little more noise and adds a little more travel time to dinner.

That makes the choice simple to think about: decide how far south you want to be. The north is convenient and busy, the middle is the compromise most people are happiest with, and the far south is beautiful and genuinely remote. Since the bridges opened you can drive onto the island, so a scooter turns the whole line into your neighbourhood.
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Klong Dao is the family beach — a wide, shallow, gently sloping bay ten minutes from the pier, with the shortest walk to a shop or a pharmacy. Phra Ae, better known as Long Beach, has the widest choice of places to eat and sleep and a row of sunset bars, and it is where most first-timers end up.
Klong Khong is the shaggy, cheap, hammock-and-reggae stretch, lovely at high tide and rocky at low. Klong Nin, further south, is the sweet spot for a lot of people: a proper swimming beach, a handful of restaurants you can walk to, and none of the crowd.

Past Klong Nin the road climbs and the bays get smaller and steeper. Kantiang Bay is the postcard one — a deep horseshoe of sand under a green hill, with only a few places to stay and eat. It is stunning, and it means driving twenty-five minutes for anything you did not think of in advance.
Beyond that is Mu Ko Lanta National Park at the southern tip, with a lighthouse, a short jungle trail and monkeys with no manners. There is also a second Lanta most visitors miss: Lanta Old Town on the east coast, a row of wooden stilt houses over the water, with no beach and a lot more character than the resort strip.

Long Beach (Phra Ae) or Klong Nin. Both have swimming, a choice of restaurants within walking distance, and enough quiet to feel like Lanta rather than Phuket.
Very. Klong Dao in the north is shallow, wide and calm, and it is the shortest trip from the pier — which matters with tired children.
In the north, no. From Klong Nin south, yes — otherwise every meal becomes a taxi fare, and taxis on Lanta are not cheap.
For a night or two if you like old wooden towns and quiet water. There is no beach, so most people visit for an afternoon instead.
More Koh Lanta and Andaman coast guides, plus the rest of Thailand's islands.