Depending on how far you're willing to go, studio monitors are the best option but they also require that you're in a room with proper acoustics.
Headphones are great in terms of the quality you get for your money but are not ideal, you only hear one channel per ear exclusively.
I only use my headphones to check for tiny flaws in my tracks which are more difficult to hear on monitors, as well as having a second source of reference on mixing/mastering for a different system.
