The main house I play in is for my legacy family. It's the "Dresden" house in Windenburg but with a lot of changes. More columns and a *huge* basement that has room for all the activities that my Sims can think of. There's a big room that is for collecting but also has several cauldrons and a séance table. There's also a big indoor swimming pool with things like hot tubs and saunas adjoining it, as well as a place for deep soaks and massages. Then there is the "experimental kitchen" (which they rarely use and is kept locked except for special occasions) that has the special stove and all sorts of things like the cupcake maker, ice cream maker, popcorn maker and the like. Then there is the "maker" room that has all sorts of skill building things like woodworking tables, archeology tables, science skill building tables (for kids), the various skill building things for doctors and scientists and the eco lifestyle things (fabricator, recycler and candlemaking.) I also have an art room with loads of art tables and easels. (That room also has guitars, violins, doctor kits, the dollhouse and toddler blocks. I have those in the bedrooms too so that they're always close to where they might be needed.) The main room is more the "entertainment" room with the piano, pipe organ, fancy bar, dance area, etc.
But if I'm not playing my legacy family I generally play a tiny home or smaller house (because of funds.) Most of the houses I've put on the gallery are tiny homes or big houses (such as my penthouse, which is meant for giving huge parties and is now my San Myshuno vacation home for my legacy family.)
I think part of the reason I like big houses is that I remember all the routing problems that the smaller houses had in TS1, TS2 and TS3! While TS4 really doesn't have much of a routing problem, I still feel more comfortable with plenty of empty space for my Sims to get around in!