Wow, that's a very difficult question! I would depend a lot on what you like to do - there's the lattitude from Cozumel's diving to Mexico City's museums and busyness to Taxco's mountain quaintness (a couple of hours from MEX by first class bus,) to Antigua, Guatemala's antiquities and reasonable prices or Costa Rica's eco-destinations. But in any case, it will be the rainy season in that entire area.