My assessment is the same as Mike Js.
With expert AI is really quite a good game. Play Italy once just to get a feel for the mechanics. Then try Germany because, the AI is generally speaking better at defense than offense. Japan is probably my favorite country overall to play.
I’m not a huge fan of playing the minors but some of the Commonwealth countries like India (Raj), Australia are fairly interesting as are countries like Romania, with the first couple of expansions.
I like playing both Russia and especially the US, but be warned you’ll often need to restart because the AI at times can clearly botch the first few years of the shooting war.
The other thing I’ll warn you about is the production system, after playing HOI IV, all the other production system you’ve seen in other games will seem silly and simplistic!