5 merchants? There is the guy who sells the Daedric hearts but who are the other 4?
To answer the question, the caves and hideouts like mentioned by Morgan make me drool. Some forts and castles are coveted by me as well but if it has to be a house, it would be that tiny little house with the secret bookshelf that leads to a a massive cave with tons of rooms and maybe water too. I think it's on the same road as the abandoned prison but you find a note in the house hinting that there is some mead behind the bookshelf and a huge payload once their scheme is complete. Anyone remember the name of that place?
I just want a house with an enchanting and alchemy table in the same room with lots of storage containers.....one for ingredients, one for soul gems, bookshelves for all potion recipes and related books. Then I want a joining patio with it's own forge, tanning rack, smelter, grindstone and smithing table on a nearby patio with several containers....one for ore, ingot, gems, etc. a container for weapons to be improved via smithing, one for 'to be enchanted' and one for all items ready to be sold and I want a bunch of rich merchants living all around me....some that are open 24/7.