The 'fastest' way (though no route is 'fast') from Berkshire will be M40/M6/M54 then A5/A548 to Welshpool, A483 to Newtown, A489/A470 to Llangurig, then A44 to Aber.
Once you get through Welshpool, the route is pleasant enough, but for a more scenic route (and certainly lengthier in time) is to use the A40/A44 through Evesham, Worcester, Leominster, Rhayader, then cut up to Llangurig on the A470. The scenery through the Welsh marches is really lovely, but the best bit is the A44 approaching Rhayader then up to Llangurig. Worth the pain of the longer journey time just to do that.
If you are feeling really adventurous, and want to experience something 'different', at Rhayader leave the A470 and turn left on the yellow B4518, then shortly take a turning right on the white road for the Afon Elan valley, which takes you to Cymystwyth and Devil's Bridge, then the A4120 which mirrors the A44 on the other side of the Rheidol valley. This is a truly spectacular route, single lane for most of the way on the white road, but with plenty of passing places and open views of the mountains and river valleys. Number of sheep = thousands, number of humans = zero.
Be careful where you stay in Aber. Lots of hotels and B&Bs, most of them grotty. Tripadvisor is a good indicator of where to stay.
Finally if the weather is bad don't even consider the Elan valley route - go the fastest way via the motorways and the route via Welshpool.