As a pagan, I simply return the same greeting I was given. I don't believe in poking my finger in anyone's eye just because I can.
To quote something that turned up on my Facebook page:
"From 1 November to 15 January there are approximately 29 holidays observed by 7 of the world's major religions.
And I don't think mine are the only ones that count.
ETA: To answer the OPs question, any expression of goodwill related to your own midwinter holiday is fine with me. "Pagan" is derived from the Latin "pagani" meaning country people ... people of the earth. At their heart, all pagan traditions - even the relatively recent neo-pagan ones like Wicca and Gardnerian witchcraft (for instance) - are rooted in actual physical places. Since no two places on earth are exactly alike, yet all are beautiful and powerful, pagans respect and even celebrate our differences. Our altar is, literally, the whole world.