The likelihood of an aurora occuring at all, as well as its intensity, corresponds pretty directly to latitude, so the further north, the better. On top of that, the further north you are, the longer the nights are in winter (up to 24 hours beyond the arctic circle), which improves your chances - and offsets the fact that auroras are more common around the equinoctes.
The second factor is cloud cover - the aurora happens far above the troposphere and will thus be completely obscured by clouds. I found a meteorological paper that shows statistics for mean cloud cover, and if I read it correctly the likelihood of cloud cover in northern Europe during winter is the lower the further east you go, but always above 50% unless you approach the Ural Mountains. The data is from the 1970s though.
Altogether, it sounds like places in northern Scandinavia and somewhat inland, like Kiruna, might be your best bet.