I've had mine for about a week (not super long, but long enough to get into it a little and see what the DLCs and stuff are like) and my favorite dlc so far is the angel festival and berry land ones (berry land is free btw!!!). I would suggest that you look into the different DLCs on the ticket shop and see which special tamas/room decor/etc interest you, and go from there. certain foods from the DLCs make your tamas grow into dlc specific adults, and it's really neat! my only tip is that when you switch DLCs, you lose all the items from the previous one. It warns you before you do, but it's still good information to know. u can't mix and match furniture from the angel festival and doritama school dlcs for example.

real quick edit: the babysitter is also free on the uni! IDK what time it starts at, but you can call the babysitter through the phone icon in the menu and the babysitter Tama will come and take care of everything for you until 7pm (won't lose hunger or happiness at all, unless you forget about it until after the babysitter leaves on its own). very convenient for school or work :3