Best is different for everyone.

Least amount of time is buying premade stuff from players, acheev vendors, normal house vendors, holiday vendors and luxury vendor on weekends in vold harbor. This will on average run between 2 and 5 million gold depending on your taste, for a 700 house like psijic.

Least amount of gold is to steal or loot recipies and do master writs to buy plans and make the furniture yourself, but beware they will churn your mats very, very quickly.

Least amount of effort is only using vendors for furniture. You have the basics at every place, each place has area specific items as well as achievements. This will in my opinion also be the most limited house because lack of options.

Personally I use a combination of 1 and 2 for my houses. I learn every recipe i can because even if you dont use a piece for its obvious use, you may find alternative uses for them. (like using khjait statues upside down for brick walls). If its a specific thing i know i need one of (telescope, clothiers form, mages apparatus, etc) i will weigh the cost from a player vs where and what i have to farm to get the recipe. More oftwn than not buying one is the objective better use of time.

Use TTC to help identify traders that specialize in furniture, their selection and pricing will be way better than a general or popular store.