For me its definitely tac bombers. When you finish your first blitzkrieg invasions, deploying 10 stack of tac bombers is really usefull to save loses vs pc players and human as well since every game you can catch many units without anti air, and casualties are massive. I find myself sometimes even in 1:15 ratio of casualties. If upgraded their range is quite sufficient and their mobility is just so useful.
With 10 stack of lvl 6 bombers you can send down battleship in 3 or 4 rounds. 3,5 dmg on naval is pretty good i think. I usually dont upgrade naval bombers since they are low on priority list. for me. I use them to scout from carriers and defend my harbors, but who uses naval bombers anyway, right
I always build on heavy defense and strategic offensive strikes with rockets, nukes or combination of some big invasion in range of nuking biggest cities , set up perimetr one province around and fire rockets and nukes - for that i also love deployin tac bombers with interceptors.If they are marching with their AA (if not SP), they are slow, so it gives strategic advantage as well, since they sometimes without SP artilery researched are not catchin up, so you can bomb cities and just march around territory with LTs or MTs together with Mech. infantry, and SP artilery, raiding enemys´factories and all kinds of infrastructures.
But bombers allways do the scouting and most of killin for me. Ooooooh, that feeling when you go scout to enemy teritory with big stack and you find some light tank stack...
If you have an air-bridge and you see enemy without AA, you can spam them at homeland, get 10 more in 1 or 2 days and in few hours they are on standby. One think sux, that you cant get them on carriers, but now i play pacific map and i am able to fly from New Zeland to Alaska and further to Mexico(theoritcaly, since right now war line is at Alaska), without need to march or send them by the sea, since our coalition controls all the islands, japan and so on. It takes 12 hours.
you can even get on the little islands in oceania.
They are cheap.
Very, very usefull, every game i try to get as many of them as i can and totally recommen to try it.