Well it also depends what country you are wanting to collect.
If collecting:
USSR: Buy either a set II or a set III. Set I has 2 good soviet tanks but thats about it. Id recommend you just buy those online or trade.
UK: Buy either set I or set IV. Lots of goodies for the UK in those. There are 2 good rares in set II but I just recommend you to buyem online or trade.
Japan: Set I and set II is the way to go for Japan. Set IV only has the Honor Bound hero. Buy that one online, im sure hes only like 75 cents. :wink:
Germany and US: Just buy any freakin set!! They all have lots of good German and American units. :-D
If you are looking to collect whole sets like my friend Sol….then just buy a whole case of any set. Simple as that. :-P