Depends on their personality! Some girls consider Chocolate the best thing ever, some girls consider World of Warcraft the best thing ever. Some girls even consider bedroom business the best thing ever (common in England, only being honest).
Seriously though it all depends on a girl's personality. Ask them, it's simple, easy, and makes you sound very cheesy and awesome. My old girlfriend almost based her entire life around Japanese Anime, most certainly not everyone's cup of tea, but it was definitely hers.
I'm speaking about things girls really enjoy and make it more obvious than most of their interests, because "Best" is a strong term and you can never be definite about it, some girls even think they know what they like best but they don't, it's a common dilemma these days.