Man, so many great pieces, been waiting for a thread like this for ages and never got around to making one
First off, the Saxaphone, trumpet, electric bass, and organ, they all look great, always nice to interact with, and actually added some variety to the musical instruments now that welcome amiibo has come around and they were nice enough to add so many wonderful items
Next, the natural low table and the natural chair. Great design overall, much-needed additions that help to just fill the gap for nice items that can serve a purpose without really fitting a specific theme, and these have wayyy too many customization options, meaning even more versatility. Hey, I've used at least 4 of those tables in my mayor's house alone.
And then there's the brown seat, a nice couch that fits in pretty much everywhere and has plenty of custom options.
I also really like the white brick wall, it's nice just to have a wall like that, pretty much able to fit into most situations and just helping to add some nice depth to my rooms in the background.
For clothing, as a Splatoon fan I loved the takadora cap as soon as they added it into New Leaf, pretty much wear it most of the time in the game since I got it, just a nice overall apperence in my opinion
Then we've got the sunflowers, I always like using those because they add a nice pop and kinda stand out from other stuff in the room, making some nice contrast.
Another versatile item I really like is the paulownia (or however you spell it) closet, a nice one-space storage unit that has space for stuff on the top, I really like using this when I'm short on space and it ends up looking nice most of the time too.
Other various items that just look Nice in my opinion, condensed into this list so it's not as redundant as this post already is at this point: skateboard rack, record box, clothes hanger, the entire alpine set, stack of books, donut box, typewriter, cassette player, game shelf, and minimalist clock, among many others. Hey, just the top of my favorites, and sorry in advance for the h u g e length of this post, just so many great items in the game, nearly impossible to narrow it down to begin with.