Grenache & Mourvedre to create the classic Côtes du Rhône blend, or GSM as it's known more contemporarily. Some of the most expensive Syrah in the world come from Hermitage, which are known for floral & smoky aromas of blackberry & grilled meat.
Typically, the French Syrah will be more restrained, while still remaining bold. Look for Côte-Rôtie, Hermitage, Cornas, Saint-Joseph for quality Syrah.
BAROSSA VALLEY, AUSTRALIA
The climate here is warm to hot, so the Shiraz here would have extra ripe flavors of cooked fruits. Close to Adelaide, the Barossa Valley has a similar terrain as the Central Valley in California. It's the birthplace of some of the msot sought after Shiraz in the world, and has some of the oldest living Shiraz vineyards as they were not affected by the European root louse.
Look for single varietal styles, or Cabernet blends.
California offers the perfect climate for growing quality Syrah, and the lack of winemaking requirements allows for varied expressions of the grape. You can find wines across the spectrum, from big punchy Barossa to restrained Rhone. Particularly look for those from Monterey, Santa Ynez Valley, Mendocino County, and Calistoga.