Where to start... There's a huge variety of terrain. My personal favourite is the Alentejo region and anywhere along the eastern border with Spain. Lots of quiet roads, big skies, ancient villages etc.
Do bear in mind, though, that it can get extremely hot in Portugal in the summer. I wouldn't ride there late-June to mid-September but the winter riding can be superb.
If you want to get more ideas the Pedal Portugal website should get you started with info on the various regions, possible touring routes etc.