Tanks are the best *offensive* land units you can buy. However, although they defend on a 3 rather than the 2 infantry defends on, you can buy 5 infantry for every 3 tanks. This means that when defending, infantry are the best. But once your defenses are in place, you will need tanks to attack.
Tanks are the fastest land unit and the most likely to get hits. Tanks are the most useful when you're the German or Russian player since you do most of your fighting on land. If you are The British, Japanese or American player, you start off with little to no tanks on the main Continents that you are fighting on (Europe, Asia, Africa). Here are some strategies for each country.
Russia- You are trying to stop the massive German war machine from breaking down the gates of Moscow, but tanks are better at attacking than defending. Build infantry in Russia help defend or build them in the Caucasas to flank them. Then, once you have a very strong defense, you can build tanks to attack.
Germany- You have the most tanks on the board on have at least one in almost every one of your territories. My suggestion is that you keep the ones in Western Europe in case the British or Americans decide to attack, put any tanks that are in Southern Europe into Africa so you can have a formidable force against the British, and send any tanks in the east off to fight Russia.
Great Britain- Build transports so you can send tanks to help fight the Germans, putting a factory in India and building tanks to fight the Japanese would also help.
Japan- Japan should focus on the navy, although it will need a few tanks to attack the allies in mainland Asia, most importantly in India, especially if the British build a factory there. If this is the case and the factory is subsequently captured by Japan, it is a good place to put tanks to attack Russia.
USA- The US doesn't have a point to build tanks if it doesnt have a navy. America should focus on building transports so they can either ship tanks to the Pacific to fight the Japanese or to Europe to fight the Germans.