Hard call for me between the IJN and Germany, but I had to go with the KM. German ships just have a certain, 'mean/beautiful' look to them. Would have loved if the US had kept the Prinz Eugen for a museum ship instead of target practice for an A-bomb.
I went with Germany, mainly because of the Tirpitz and Biz and the fact in my old gaming group everyone always chose USA right off the bat and I was stuck with the Axis. Some great battles tho.
Mine is italy followed by the UK , I really like the close escort objective rush swarm builds and both have alot of interesting units that fit that.
Historically the med is my favourite area to read/learn about. I do really like playing as almost every nation maybe France the least but that is more because lots of people in my group like France as their beloved underdog nation so I have focused on italy as mine.
I took Japan, because I like air power and tough matches (US is cheese). A very obvious soft spot for France (I wish we could have developped the programs)
I was fairly fond of the French. Fighters good enough to be fodder, fairly decent BBs, and the destroyers are surprisingly useful and quite exciting. I try to stay away from the fantasy/what if ships but the Japanese and Italian cruisers are quite fun. I've never had much luck with the British and my American fleets always consist of bringing 50pt BBs against the superbattleships and sinking