If looking for comfort close to the national park gates and the Sok River, it would be difficult to do better than Tree Tops.
Check rates at Tree Tops River Huts on Agoda.com
It actually has an entrance directly in to the Khao Sok parking lot, beyond the front gates of the park, so it's literally a stone's throw from the visitor centre. The resort is also situated right beside the scenic Sok River in view of the Khao Sok mountains, so its location is optimal in more ways than one. Accommodation gets high marks as well. It's surprising they chose the word huts in their name as the rooms on offer here are big, comfortable, and constructed mostly of stone. They offer a few different types of stone cottages and the price varies on amenities. The cheaper wooden fan cottages are set back from the river but are spacious, bright and comfortable with a cheerful atmosphere in three different styles. Stone cottages, some with air-con, fridge and TVs and others without, are larger and exceptionally clean and cool with some great porches perched directly over the river. The river view rooms are well worth shelling out the extra baht for those shaded balconies. There are also a couple treehouses and a large family cottage on offer, and all rooms come with hot water. The steps up to some of the rooms are steep, so not ideal for those with young kids in tow. There's a restaurant and tour company on site as well, and staff are laid back but friendly and speak English reasonably well.
Type of room, low and high season prices
Room: Bungalow fan private bathroom, low season: 400 baht, high season 400 baht. Notes: Larger rooms 700 baht
Room: Bungalow air-con private bathroom, low season: 1,000 baht, high season 1,200 baht. Notes:
Room: Family room, low season: 1,600 baht, high season 1,600 baht. Notes: Sleeps six