It depends a bit on how far you would like to travel each day as to how far 5 days might cover and, if you are B&B-ing and relying on public transport, whether it all connects. Some days can be quite long because accommodation is a little scarce along the way.
Mrs GP and I did the southern half a couple of years back, from Chepstow to Churchstoke. Our favourite section was that between Pandy (near Abergavenny) and Churchstoke/Montgomery, taking in Hay-on-Wye, Gladestry, Kington and Knighton. This is around 70 miles or so, and took us 5 days, containing what we considered to be the best walking and views, and the most variety. And, apart from the towns, it's also pretty quiet!
Abergavenny is well served by public transport, as is Welshpool, along with Kington and Knighton. Other places have some connections too, but can be infrequent or slow.