Depends on what's important to you, because anytime of the year is a great time to go to TDR.
February is a slower time of the year but it's quite cold weather in Tokyo, so if you don't mind the cold weather, then visit around that time. If you can hanbdle the heat and do not mind being packed like sardines in a 4 hr standby line for Hunny Hunt, then go in August. If rainy weather does not bother you and do not mind seeing all of the varied live outdoor entertainment cancelled due to inclement weather, yet you get a manageable park with lower crowd..then go in June or july. If you like your disney park experience to be themed and festive, with extra special entertainment and unique decor, while putting up with a bit of the crowds... then go in sept/oct for Halloween, or nov/dec for Xmas Fantasy event.
Lastly... if you want a little romance to go with that bitter cold weather, then go in february and enjoy Princess Days and Cinderellabration: Lights of Romance at TDL, or go to TDS and enjoy this year's new winter event "Season of the heart". It will sure bring out the Disney Princess in you!!
Good for you Treat but don't over look heading out at Christmas or Halloween as this is simply a great time to visit.I visited in Sept for Halloween and while the waits could get long for some attractions I still got loads done and it was fun to see all the Halloween stuff.Weather was great too not to warm but warm enough.