Avoriaz has a good snow park, with several parks located around the area. It also has the burton stash located in les lindarets, just down the over side of avoriaz, which offers lots of natural terrain park features, and lots of natural rails. The burton stash only uses wood features to reduce the environmental impact of the natural park.
The park situated in avoriaz offers 3 jump lines, green, blue and a red line, with features that commonly change integrated into them, such as rails and fun boxes.
The Arare park located on the high peaks of Avoriaz, is only a 5 minute ski and lift away from avoriaz, with a selection of three lifts to take you, however these lifts can get extremely busy in peak season.
The Arare park offers a black jump line and snowboard/ski X.
I was recently there and the local park snowboarders had recently constructed an enormous lip.
The Arare park offers the more challenging features and jumps, and stays in good shape during the spring due to it's altitude and cooler temperatures.
There are more parks scattered all across the Avoriaz ski area, with another park also in les lindarets, offering more rails and one relatively small triple line.
I hope this helps, and am sorry i can't comment about Meribel.