HOI3 without patches or expansions was a complete disaster, and almost unplayable. The concept was very admirable, and I seriously wish that Paradox had invested the time and effort into getting it to work as originally intended, but the AI had a few fatal flaws, and the fans demanded a working game NOW! (Can you blame them, after spending money for a game that crashed or went brain-dead less than half-way through the war?) The developers ended up ripping out the entire diplomatic engine and scripting everything to get it working, which turned it from a "historically based sandbox" into an "on rails" campaign game. HOI4 actually had a MUCH smoother initial release.
The 1.3 and 1.4 patches brought HOI3 up to the point where you could get a war that resembled WWII about as often as not. Without the patches, it's "interesting" in a way, but too frustrating to play and have it go belly-up after you sink 10-20 hours into setting up your army and economy, only to have the rest of the world "park", or your main ally send 95% of its army to some forsaken corner of the map to starve (Japanese invasion of Finland, German amphibious assault on India, etc.).
Semper Fi, the first expansion, fixed several major bugs, and I consider that the minimum needed for a good game. The final patch for SF corrected a huge oversight where the Defensiveness and Toughness of land units didn't affect combat. Air units were NEVER fixed. For the Motherland and Their Finest Hour add numerous features, including a "wargoals" system to demand territory without total annexation, expanded convoy and lend-lease options, etc., but further restricted some of the diplomatic and espionage options in favor of more "gamey" tech stealing, coups, and underground resistance cells. Note that each expansion requires all of the previous ones, so if you get TFH, you'll also need FTM and SF.
So far, in my opinion, HOI3 is currently the better game, but HOI4 still might turn into something really good with time and a few expansions, or not.