It can be very situational, but I enjoy Pan-Asian the most. Any other doctrine feels slow. My favorite unit, Heavy Tanks doesn't go well with Pan-Asian doctrine though. There are many maps where Pan-Asian isn't a choice however.
If there is an extended peace period Allies are definitely good, as they research better and upgrade better than other doctrines. Good for less active players as well, and they have the best air force.
On smaller land maps, or perhaps Antarctica map, I like Axis for Rail guns and starting unit strength. Those Labs on the Antarctica map provide lots of resources for Axis as well. Axis Railguns can be oppressive to play against if there isn't a lot of ground to cover on mostly land maps. Stacks of 5-10 Railguns, 5-10 anti air with a couple motorized are very difficult to deal with unless you can trap the railguns into a close combat somehow.
Comintern does well on larger maps, as armies get huge their upkeep reduction shines. I think their SPRA is amazing, and between its speed and unarmored attack it can take out half the enemy army in 4-5 hours while they are asleep.
