Everybody has their own play styles that work, especially depending on doctrine. Personally, I normally use the communist doctrine, with a heavy emphasis on Rocket Artillery and Air Force.
My goal is to build many small armies of 10 troops all featuring SP 1 anti-air, and then a combination of medium tanks and motorized infantry. Mechanized is also very good. Most of my armies are accompanied by an artillery regiment featuring rocket artillery (SP or Normal) and SP Anti-Air.
Then I build 2 or 3 air forces consisting of ten units, mainly Tactical and Attack Bombers and 2-3 interceptors. I try and get everything to around level 3 or 4. The combination of Air Force and Artillery allows me to inflict casulties without engaging my ground troops until it's necessary. However recently I've found that Air Force's usefulness is decreased in late game scenarios when large army stacks have great air defense capabilities even without anti-air.
For Navy, I rarely invest heavily in my navies unless I'm a naval power. I even more rarely use battleships. Normally, I build 1 navy of 10 ships, 5 destroyers, and 5 cruisers, all highly researched. I find it's faster, cheaper and almost as good as a more battleship and aircraft carrier-based navy. To defend my land if I'm exposed to the ocean I use a combination of submarines and naval bombers, as well as rockets if I have some.
While I think meta's demand on your personal playstyle and doctrine, I think there are some constants across everything. First, I think it is necessary to invest somewhat in your economy. It's difficult to raise large or highly-advanced armies if you're running out of resources every day.
Second, I think you must always research based on your strengths and smartly. It's very important to research. While it might seem smart to build massive armies of cheap, easy to produce and disposable level 1 infantry and level 1 light tanks. But those armies, even in the communist doctrine, will not stand a chance against smaller, high researched armies. It's important to research, but research smartly. For example, if you're communist you don't need to research to crazy high-levels, but it's still important to research. Additionally, don't go crazy on stuff you don't use. For example, don't research paratroopers if you're not gonna use them. Use those resources somewhere else. Personally, I mainly research motorized infantry, rocket artillery, air force, and medium tanks. Then I do some work with cruisers, destroyers, infantry and light tanks and anti-air. Then I don't touch much else.
Just be smart and do what you want. I think what makes this game fun is the ability to do what you want and the flexibility it provides.