Really hard to say.
In summer, this baby: Store Midtmaradalstind, found in the Hurrungane massif of western Jotunheimen, Norway (actually they're all in Noreg, sorry to be unpatriotic )
In winter, it has to be a mountain very close to the last one, and considerably more famous: Store Skagastølstindane, famously first climbed by WC Slingsby. In winter this route is transformed, the glacier climb is soft but always exciting rather than dangerous. The final ice-rimed rock becomes very tricky in places - and that view! Words cannot...
I can't find a decent winter pic, so here's a summer one to give you at least a vague idea:
It's really close, but I think I have to go with those two
Another vote for the Buachaille here, for two reasons: firstly, when you see it, you're really getting into the Highlands, which is always exciting; secondly, when you see it, you've made it over Rannoch moor, which is always a relief.
Why do cars always break down in the middle of Rannoch, at dusk, in the rain?