Usually Barcelona - has a bit of everything.
Silverstone International is the great for making quick tests, as I know it inside out and its very short, meaning laps can be pumped out quickly, yet still has a good variety of corners and a long enough straight.
Mugello I love as a track, but is a lot harder than average on tyres, so isn't always ideal for testing.
Quite like Paul Ricard for testing too, some interesting corners plus a super long straight.