It depends on my mood. Most of the time, I like to play by myself because I don't have to worry about things being stolen or being yellwed at for breaking Town Rules =P
Well when you first get the game it's better alone when you're trying to pay off the mortgage and all. But once your mortgage is payed off and you're rich, it's a lot more fun on Wi-Fi.
i do my usual things (making sure flowers aren't dying, buying everything i need from nooks/ables, picking fruit, make my snowman for the day, dig up fossils/gyroids) and then when i get bored of fishing i'll check and see if anyone's got their towns open. if they're not, i'll usually buy all the paper from nook's and send 4 letters to my town animals, and then 4 letters to people i've wifi'd with that i haven't heard of in forever... but i don't hear back from anyone. =*( i feel like my town is falling off the face of animal crossing world, lol.