Try them all, early in the game it's easy enough to change trees/nations... It's not until about tier 4 that it really matters.
Personally I like the US Tree, big guns.
France seems to still be a bit OP if you feel like grinding through the tier 1-5 to get to them. Germany is a good balanced nation, unfortunately that doesn't make them the superior tanks. Russia is the same, just more armor, more speed.
The thing about France is they have post WWII tanks with auto-loaders. So most French tanks have the ability to fire rounds at crazy speeds but then have a long reload.
What you need to decide is what tree you're most interested in. SPGs are the artillery, they have biggest guns, longest reload, virtually no armor. Tank Destroyers have the second biggest guns, they don't get decent armor until the later tiers, and some move quick some are the slowest in the game. Scout Light tanks only go up to tier 5 (up to tier 8 in French line) they are easily the most maneuverable and some of the fastest, but obviously they don't have the best guns or armor. Medium Tanks are fairly quick and maneuverable, and most can take a few hits. Heavy tanks are even less maneuverable, but they have bigger guns and the best armor.
Each nation does each tree a little differently, but they all have their pros and cons. But it all depends on your playing style. I have tank ADD, so I want to try every tank in the game... So although I've had my tier 10 researched for months, I spend all my credits on trying new tanks.