I would certainly second the suggestion of the Northern Carneddau.
The only real issues, is the much shorter daylight hours, with darkness descending on us at around 4pm, or slightly earlier depending on weather conditions.
Not that it will discount any venture into very remote regions of Snowdonia, but if you can park your transport above the main Abergwyngregyn Aber falls car park, near the power lines, its possible to reach Foel Fras, and back in around four to five hours.
In the depth's of Winter, this would make a great introduction to Winter walking.
Obviously you would do the walk in the company of someone more experienced, but the Northern Carneddau, are like a glorified version of the Brecon Beacons.
Well defined paths, and in good visibility, very easy walking.
Even in well settled snow, the traverse of Foel Fras and Drum, from above Aber, is within most walkers capabilities, even a total novice.
There are no sheer drops, or any uneven pathway's, just good solid ground, amongst some of Snowdonias remoter countryside.
If your blessed with one of those glorious cold sunny days, with high cloud, the Northern Carneddau come highly recommended, but its not an area to explore by yourself, in the depths of winter, as its very remote, and desolate.