I use masonite panels (waterproof hardboard). I prepare them with Lefranc et Bourgeois gesso. This type of acrylic gesso has a very subtle tooth to it, which simplifies smudiging out brush strokes.
Occasionally I also use mdf panels. Drawback is that when a (water-based) acrylic ground is applied on this, it raises the 'grain', ie. the wood fibers fill with water and swell up. The surface then isn't smooth anymore..