When searching for party wine you want to be looking for a simple easy drinking flowing wine, your not asking it for intense complexity but just to please your guests. The grape varieties I would lean towards are Sauvignon Blanc, Viognier and unoaked Chardonnay for whites and Pinot Noir and perhaps a Merlot blended with some thing like a Cabernet Franc. Light and fruity with little or no tannin.
Whilst these grapes have been chosen for a selection of party wines the same grapes are often produced to make complex wines and this is reflected in the price.
The areas of the world I would search are places like Chile, Argentina and the Vin de Pays of France, here you’ll find great wines at good prices.