Risk: Global Domination is the official port of Risk.

Pluses

• Cutesy graphics
• Lots of maps (though most are paid DLCs)
• Play with friends (pass and play or online)
• Play with random people

Minuses

• Ad driven until you buy the starter pack ($10)
• Map packs are 4 or 5 maps for $5
• UI could use some tweaks for faster gameplay and to prevent accidentally going skipping a turn stage
• Some starter maps require coins to play each game and I hope bought map packs don't need coins

It's an ok implementation of Risk.