Pick whichever interests you the most, none of the nations are really that much better then the rest. I choose the Germans because of their ability to fight off the Romans and I have some German blood. I also like how smart they were and still are! Most people pick the Romans because off the extra troops you get for multiplayer battles.
Oh and the Germans get a buff to the "Rally" ability in battles. Less time for cool down and also the troops start in the Rally mode longer so that they can reach the rally point faster. To me rally is important for a battle.
It depends on your style of play ... If you are offensive, Romans are a good choice (bigger army), German too if you want to have a better of your troops ...
Just look at the characteristics of each nation and make your choice according to your personality. Do you want to beat every player you see, or do you want to have an indestructible base ? It's up to you