1. Luang Prabang
I just love the part of old city between the Mekong and Khan River. And good restaurants with good western food (and not expensive). Easy to get there by bus from Vientiane or by airplane from Chiang Mai, Bangkok or Vientiane (and other international places). Great place to make day-trips in the area.
I love small cities with history, the "Wat Sisaket" is my favourite place to visit. Just walking along the riverside. And of course good restaurants.
3. Don Khong
One of the 4000 islands in the southern Mekong "delta". A village in between a beautiful natural area and with a remarkable history. Beautiful old colonial houses.
4. Muang Sing
High up in the north of Laos in the mountains with almost untouched nature. I love mountains.
5. Vang Vieng
OK I do not like the "khao San road" area of town but it is easy to avoid and what is left is a beautiful karsten landscape and friendly people combined with good French food.