TAD - China for both offense and defense. Good economy, and if you build three layers of walls before sending Great Wall the build time increase isn't much of a problem plus you end up with walls/gates that have 17k in hps(more than a european fort) if you use improved buildings as well.
Asian castles are more effective than european outposts, having 1k more hps for only paying 100 gold over an outpost to build them(1 Flatland Castle adds +10% attack, and hps while Extensive Fortifications adds +4 build limit and +25% hps on top of the +40% hps from Improved Buildings as well). China also has Land Reforms, everything(except walls) build 50% faster and you get more experience upon a buildings completion(+100%).
Well, it may come as a suprise, but Russia is statistically "the best" at defense.
First, having instant infantry really makes you civ OP, and the Dueling School Card+Fencing School+Standing Army does just that.
Second, National Redoubt is like SO OP it makes me mad. Musketeers building forts? Nit even fair.
Third, Sevastapool Reforms allows you to build a 5 layer wall in 2 minutes around a maxed out city (4 plantations, 4-5 mills, 2 factories, ect.) That is so unfair, if you play against Russians for a long game, the walls are just about touching yours from across the map.
Offense? Well, everyone can do that. It is the player who is the best. That would be iamgrunt.
What if the Hokey-Pokey really is what it's all about?
The russians are also very good on attack because if you build a fort next to the enemy base you can instant spawn musketeers and make other units very fast. also you can use opriniks to crush walls extremely fast with insane seige damage(especially with advanced arsenal) and then destroy their eco with 3x bonus vs settlers. Cossacks are also quite good because of the fact that they take only one population each. Grenadiers can be effective, but as it has been said, dont use stretlets mid to late game, they are just too weak and are only good for a mass unit rush very early on.