I normally don't play favorites, but I thought I share with you my current day-to-day white wine preferences. These are wines of excellent quality at $20/bottle or less prices.
When it comes to Riesling, Polish Hill in Clare Valley is an exceptional subregion, mainly due to Grosset. But if you don't want to fork out that kind of money, there is O'Leary Walker Polish Hill Riesling. This is an excellent wine for less than half price.
If your choice is Semillon, an easy to drink wine is the Mount Pleasant Elizabeth. I would not drink this from every vintage, but the current wine comes from the excellent 2014 vintage.
Finding a sub $20 bargain for Chardonnay is harder, but I experienced two excellent wines in this category. Both are difficult to find, though. The first is the Pfitzner Chardonnay. It is a grower wine from the Adelaide Hills. Its fruit has gone into the Petaluma Chardonnay, and its style is similar, maybe a little fuller. The second is the le Versant Chardonnay from Southern France, outstanding for its price.
All these wines are in the citrus spectrum and basically dry, with maybe a tiny sugar residual. I recommend them highly.