For a first trip to Wales, wanting to cover a small area of scenic beauty, I'd go for Snowdonia. But just remember, all those hills generate their own weather, so it can often be dull, wet and dreary there when the sun is shining to the south and east. Noggy's pics show that mid Wales has it's great scenery too, but I'd still say Snowdonia for your first visit, weather permitting.
Great scenery through the Llanberis Pass (can often be slow traffic though), with the visitor centre at the south end, Pen y Pass, could ride(?this a bike trip, or car?) up to Caernarfon with it's magnificent Edward I castle masterpiece. Not too far from Conwy for another impressive castle, or A5 down to Betws Y Coed is a lovely road past Tryfan; Betws is a bit touristy but good for fish n chip stop, great views (again, weather permitting) of the Snowdon Horseshoe just beyond Capel Curig headed west towards Pen y Pass (A4086). A470 from Betws to Ffestiniogg is another nice one. Horseshoe Pass further east and down to Llangollen with it's steam railway and motor museum (haven't actually visited the museum myself) and some great roads down from there to Bala, and another favourite of mine, the B4391 down through Llanfyllin.
Don't forget if time permits, Cadair Idris and Tal-y-Llyn off the A487 between Machynlleth and Dolgellau further south, and the A4212 between Bala and Trawsfynydd back up towards the north again - a brilliant road for bikes in good weather.
Plenty more too