Bagus Bay Guest House is without the doubt the most touristic place to stay in Samosir Island, which makes it one of the most touristic places in Sumatra, even during the wet season. The location is amazing - the guesthouse is located just next to the ferry pier, so it is really convenient to get there from Parapat. We were a bit surprised to see that there is no aircon in the room - and relieved to realise that it's not necessary! The room had a very pleasant temperature throughout our entire stay. The photos don't do justice, in reality it has looked much better. Our only complaint would be the mosquito net, as it was not possible to cover 100% of the bed, there were always some holes, albeit small.
The restaurant in the guesthouse is quite expensive by the local standards, but it's one of the best in town. Try to stay there on Wednesday or on Saturday, as there is a showcase of the local Batak dances and music.