Arran provides some excellent mountain walks. Goat Fell, its highest mountain is the favourite but this is also reflected by the muddiness of the route. If you only have time for one walk you would do far better to do a circular [sort of!] route from Sannox Bay with the magically shapely Cir Mhor as your focus going up the ridge on your south side and back over the one to the north. This is quite a long hard walk but the scenery is outstanding.