Hey, I would suggest between September to November, pleasant weather not much winter clothing required. You can visit now also as winter just started.
It will be nice.
Before march after October is a best time to visit and after august till October is another best time to visit. Summers have cool weather still heat prevails in Ooty.
If you want to experience the complete chill and Misty ambience then I would suggest from October to January. But if you're about to visit from January to March then it is the best time to witness the greenish landscapes and beautiful flora of Ooty hills. And don't forget to visit the Ketti Valley.