There’s nothing more Scottish than a sturdy stronghold in a scenic spot and the country’s studded with them.
From its ruins to its near-palaces, there are well over 1,000 castles in Scotland and Wikipedia has a pretty comprehensive list here. These are way too many for the average trip (!) so you’ll want to concentrate on the best. Visit Scotland, the national tourist organisation, list the following top 13 here:
Certainly, it depends on your criteria: Stirling and Edinburgh are both grand and magnificent; Eielan Donan full of Highland magic, as is Castle Stalker and Duart Castle. Dunottar deserves to be on the list too, for their sheer moodiness as does atmospheric Urquhart and spooky Kilchurn Castle.