Mudwars.io is an entertaining io game with tank upgrades
You have to tunnel around the map to farm precious gold ores and earn points. Every time you level up, you can spend a point on one of the many upgrades you have to choose from, such as weapon damage, reload speed, etc. Some of your special abilities in Mudwars.io allow you to become invisible and sneak up on your enemies. It can be a really effective strategy. Give it a try :)