Usually the best time to visit Scotland is from mid April till the end of May but you can be lucky and experience good weather all year round. April is great with the patches of snow, brown grass and blue sky contrast whereas in August you can experience nice green/pink colors. Can be crowded during weekends with good weather but it is still memorable experience.
Beinn Alligin means a Mountain of Beauty in Gaelic language and I must agree with that. It consists of two tops - Munros (the mountains of more than 3000 feet in height). The walk begins in a small parking lot near Torridon village (see the link below for more information). The walk up the first hill is very steep and it feels like a stairway. After approximately two and a half hour you should reach the first top with beautiful views (if you are lucky and the top is cloudless). You can see Liathach ridge and Beinn Dearg to the east and Baosbheinn to the north. Care is needed to walk down to the bealach to reach the second top, which is higher than the first one - 985 meters above sea level. The views are exceptional with the last part of the ridge to the east. This part is called the "Horns" and if you want to traverse it prepare yourself for some scrambling. It is not hard but one should be careful. The descent is quite steep so be careful also on the way down. The whole ridgewalk is approximately 10 km long and takes 7 hours to complete.