Quite agree, the Poros-Sami road is a lovely drive, as is the east cost route up to the north of the island with awesome views over Ithaka. The main west coast route... I find it a bit of a challenge - stunning views but you really have to watch the twists and turns in the road (and the little rock falls).
Drive around the lagoon from Argostoli to Lixouri is also quite pleasant. Come to think of it, the drive from Poros to Katelios, via Skala, is quite nice, too.