There are a lot of graphic sets/styles that have been made for NeoLemmix. Picking just a few favorites is hard, but I quite like namida's Clockwork and I recently started making a few levels with Arty's Underwater set. Some tilesets are suited better to different things; Gigalem's Freedom Planet tilesets have a lot of big pieces that are good for big levels.
It's not that hard to make your own style too, but it does take a while. I suggest trying to build levels with a variety of different styles to see which ones you like best. Don't be afraid to mix tilesets together too! People like Strato Incendus have showed that you can find some really neat combinations sometimes.