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).

For TNO these are the SocDem/socialist paths and their respective flavors:

• The US can go RFK in 1964 who epitomizes the "SocDem hawk" American archetype. I believe only the difficult "very radical LBJ Great Society" has more robust and comprehensive reforms than a two-term RFK. However it can be relatively easy to get RFK assassinated if you don't know the trap paths. An RFK opening means you can pursue several other neat paths - assassinate RFK before 1968, you can elect Michael Harrington (OTL founder of the DSA) in that election; an RFK opening is practically a requirement to elect L-NPP in 1972; if you want a more centrist but interesting alternative you can elect John Glenn in 1968 and go to Mars
• Electing Gus Hall and L-NPP in 1972 is very difficult and requires a specific set of actions along the way to polarize American society to extreme strain. You'll need to deliberately sabotage things to push up L-NPP support above 20% in order to make them eligible for nomination in 1972. This is considered the second-worst outcome for the USA because you've effectively pushed American society to the brink and then installed a President who's going to try and impose communism from the top down.
• In Britain if you win as the resistance you can elect Harold Wilson (the OTL 70s Labour PM who the military seriously considered couping) and the SLP, abolish the monarchy, and build a comprehensive welfare state. Theres also a Hardliner path that's less pleasant and unfinished as yet.
• The other communist paths are all in the Russian warlord states. The most straightforward is the WRRF, the direct continuation of the Red Army resistance to the Nazis. These guys are a great starter nation for a newbie, as their mini-game is straightforward, they have their backs to a coast, have a slow and gradual reunification tree that won't get them swarmed prematurely, and they get strong military bonuses as a handicap. They have two paths - Zhukov and Tukhachevsky. Zhukov is a reformer who can be succeeded by either a military-first guy, an orthodox centrist, or the SocDem Yakovlev. Tukhachevsky thoroughly militarizes Soviet society in preparation for world revolutionary War - i hear hes one of the bleaker paths. Zhukov WRRF is the Canon Russian unifier for the future 1972 bookmark.
• Also in Western Russia is Komi, which is basically the "clusterfuck paramilitary Weimar Republic". It can go every ideology except one. The three communists are Mikhail Suslov, Andrei Zhdanov, and Svetlana Bukharina. Suslov's is pretty boring - if you're a fan of OTL Brezhnev-era orthodoxy, he's your guy. Zhdanov is the "Ultravisionary", meaning he wants to do Space Communism and pursues a lot of insane (and often unethical) weird science projects that are REALLY GOOD IDEAS. Bukharina is basically "Libertarian Soviet Democracy communism but not always nice and clean" and is probably the most realistic LibSoc path in the game.
• If you want to do one of the "funni" paths (there are many in Komi) you can do Ivan Serov, who starts as a communist but goes off the deep end to literally become NazBol with his "Ordosocialist" ideology. He apparently can get some insane military bonuses versus the Germans
• In Western Siberia you have Kaganovich, who pursues collectivization and mass industrialization like OTL Stalin. His path is actually pretty neat. If you go down the industry tree for him you can get a truly insane number of factories; if you go down the agriculture tree he does mandatory government-issued gfs. Kaganovich can later get couped by Cornlord Khrushchev if you want to do revisionism. The other west Siberian unifiers are Rokossovsky's military junta which can stay a military junta or do Yeltsin liberalism (cursed); and Omsk, which is one of the most horrifically bleak paths in the game if you're up for "funni" shit.
• Central Siberia has a lot of options. Tomsk is a liberal republic centered around artists and intellectuals, you can elect SocDems there. Vasilevsky is a splinter unit of the Red Army occupying western Mongolia that recently got content. Then there's the Siberia Black Army that does libertarian anarchism, but has to carefully balance this idealism with security concerns to avoid becoming a military dictatorship. I've yet to play them but hear they're wholesome, they can literally crowdsource nukes which IMO sounds like a Really Good Idea. Finally there's Kemerovo, where another former Soviet general has gone mad and declared himself King Rurik II as a throwback to Kievan Rus aesthetic. If you go with his son as a successor you can do socialistic monarchy with strong labor unions and democratic institutions.
• Finally in the Far East you have a few more options. First there is Sablin, who is basically "wot if Gorbachev but the underdog". He has basically become a meme because the path where he stays true to his ideals is wholesome to the point of being diabetic. He can also get corrupted to an extent if you compromise on those ideals too much in the name of security and short-term gains. Sablin can be tricky for a newbie because he starts the game at war with Yagoda with inferior troops, so you have to do tricksy maneuvering to win.
• Then there's Yagoda, who has constructed a horribly repressive NKVD police state. He also has the most legitimate claim to being the successor government of the USSR as he oversees the rump Presidium that fled to the east. He can build a personal dictatorship that does Dengism or potentially get couped for a slightly less repressive orthodoxy.
• Finally there's Alexander Men, who is not around at the start and only consolidates the backwater Siberian villages around 1964 (you'll be prompted to switch to him if you want to). He does "Christian anarchism" and while not explicitly socialist is a pretty nice guy all around.

Another interesting path in general thats emphatically not socialist is doing a Speer-Gang of Four run in Nazi Germany. In this path Speer is basically Nazi Deng and comprehensively reforms the economy, most prominently by trust-busting and giving the slaves slightly better lives. But if you let the Gang of Four (who are the real reformers and are represented by three OTL West German chancellors and a Wehrmacht general who OTL was part of the July 20 plot) get the upper hand by the end they can turn Speer into a puppet figurehead, make peace with a massive slave revolt in Eastern Europe, and effectively dismantle the Nazi dictatorship and reintroduce a semblance of democracy.

In TNO, Tyumen, led by Kaganovich is the successor to stalin and Sablin in Buryatia for the idealistic leninist revolutionary. For kaiserreich, the Commune of France and Combined Syndicates of America are the most fun leftist countries.