Depends on your experience/skill level, and in the case of TNO, what you're looking for.

For Kaiserreich, the Commune of France is the best one to start with. The mechanics are simple, extremely straight forward, and YOU dictate where you're doing, which revolutions you're going to support, your plan to destroy the germans and so on. Russia/Soviet Union have a very similar gameplay and is also good for learning, although it can be hard to manage it for a first playthrough (horrible industry at the start, and the debuffs). The CSA is extremely fun, but it's best kept for more seasoned players, considering how hard the 2ACW is if you don't know what you're doing. The Union of Britain has a similar gameplay to France, but with less agency, since France will always declare war on Germany in 39; also you have to deal with the Internationale's navy, and manage the FUCKING FUEL THAT KEEPS RUNNING OUT. Learning naval mechanics is excellent on the UOB, but as I said, for a brand new player, France is the best. The last update also reworked the Ottomans, but they're a bit finnicky to handle imo.

For TNO, it's less about gameplay, and more about the narrative. I personally played a few presidents for the USA, Heydrich and Speer, left-wing italy, democratic Iberia, China, """Free China""", and a few socialist russian warlords. And Burgundy. Besides the last one that made me die inside, I found all of these extremely enjoyable to play, considering most of them (bar heydrich and Burgundy) are "good" playthrough, where you take a nation, and unfuck it. Iberia is relatively chill for a new player, but you'll run into problems dealing with terrorism later on. Italy is probably the best starting nation for new players, the UI isn't hard to grasp, you have an economy, a good army, a few puppets and some wars. On the reverse, the US playthroughs are the hardest, bar none. The election mechanics are so cancerous I resorted to cheating to enable all decisions so I could get the electoral outcome I want. If you decide to power through the RNG-based electoral system, know that you'll definitely feel the entirety of the American Bourgeoisie breathing down your neck, no matter what you do to help the common man and end segregation. (Or you can go the other way, and do some gamer stuff in the US to permanently destroy it).