In reply to Costadia:
Hey
Not sure about public transport, but a car will undeniably help - the best routes always seem to start at some point in the middle of nowhere, I'm especially thinking of the Reinigeadal area, the long road out west across the North Harris coast, Pairc Forest (further afield)...
The four big hills - Cliseam, Uisgneabhal, Oireabhal and Tiorga Mor are all great - they feel like legwork to link together with the gaps between. There would be some good low stuff to be had by walking under Sron Uladal, the old path to Reinigeadal..
Best attribute of Harris imo is it has lots of absolutely awesome smaller hills: Todun, Sgaoth Aird, Beinn Dhubh (Luskentyre), Ceapabhal, Roineabhal etc etc
A lot of these hills are actually really short trips but absolutely packed with character, so I've had a couple fun summers on Harris driving around, nailing out hills on a one-on-one basis, and packing in as many in a day as I can.
Through routes can be hard to do with the nature of the terrain and place, so I've always found shorter routes more natural.
Two areas slightly further afield I'd like to get to sometime, and have yet are Pairc and Uig.
Pairc is just a wild little peninsula - Caiteseal or Uisinis from the nearest road would both be absolutely epic - probably best backpacked over two days. The Uig hills (Mealaisbhal, etc) just look lovely.
That's all quite hill-orientated, but hope it helps
Kevin