Hmmm.
When I think of offense I think of 4 *elite modern armor as the best army.
However, in practice I rarely get to build these, since I am usually in solid control of the game by then. I would say defensive armies are the best. Most recently I have had armies of 3 infantry, later adding an MI, or armies of riflemen with infantry as the fourth unit.
The AI avoids attacking them, so I put them in the front line cities. When the AI attacks it goes right by them in an attempt to reach undefended inner cities. Of course it never works, as my Huge stack of artillery redlines any incoming units which are subsequently destroyed by cavalry, tanks, whatever is available. When it is time for offense, such a defensive army is great to defend my arty stack, as the AI cities are bombed from metropolis to townhood and the units are redlined before the assault starts.