You should try the light tank. It's fast, powerful enough to do quite some damage and has enough armor to keep you alive a good while. You just have to keep moving a lot, and know when to retreat.

But again, NOD isn't for frontal assaults. Unless you have artillery or other flametanks backing you up, your flametank/light tank is going to get stomped.
Hit and run tactics, that's what it's all about

Stealth tanks can be quite deadly and fun, especially if GDI sets up a tank barrage (ie: they have powerul/lots of tanks backup with engineers, making them close to invincible), because in a heated battle, no one will pay attention to you (and their visibility will be greatly reduced by the explosions and stuff) so you'll be able to squash all their engies and they won't even notice. Don't rely on its firepower though, stealth tanks are relatively weak...

You should use mobile artillery if you have good aim and like picking off enemies from 3 miles away. It's awfully powerful, precise, but slow and weak, so you can't just pop in the middle of the battlefield and take people out.

If you really want to go with flametanks, then you have to be smart and take paths where you know they won't see you/shoot you. If you can get close to the enemy with some armor left, they're dead and buried.