On Shimla - Kaza - Manali route Lahual amd Spiti is beautiful every where while on Leh route it is again so beautiful lile Keylong, Sissu, Jispa Baralacha La etc..
The most beautiful part in this route is Gramphu to Kaza. The route is awesome specially in May-June when you can see thousands of waterfalls around and snow is still there. One night @ Chandratal lake is must.