There are lots of options and opinions, but popular choices are the Hornady V-Max and the Sierra BlitzKing, both "ballistic tip" designs. The combination of extreme accuracy and quick expansion makes them an excellent choice. Others include the Nosler Ballistic Tip and the Speer TNT. Generally a very light (40 - 50 grain) round with high velocity (3300+ fps) make the best varmint rounds.
For larger, tougher varmints, such as coyotes, the 60gr V-Max is an excellent choice.