My Dad has played a ton of Romance of the Three Kingdoms over the years (and I've played a bit as well but not as much as him), and each one has it's own ups and downs. I don't have much experience with RTK 1 - it feels pretty dated (especially the interface) and really slow-paced to me though. RTK 2 (my Dad's personal favorite) is basically a very streamlined, fast-paced version of the first game. While, as a result, it's not the deepest the series has to offer, it has a lot of plain old fun value and enough variety to stay entertaining. Once you get down the basics, it also has a "pick-up-and-play" feel that few turn-based strategy games achieve. RTK 3 goes the opposite route of being much slower with a lot of focus on building up your cities and armies and really managing your empire, which gives the game a much greater sense of scale. As you noted, it's more detailed, and it was also the first one to add troop variety (cavalry and crossbows were introduced) and a robust officer creator system, which always makes for a ton of fun (RTK 2 only let you make 1 new ruler with 1 officer.) RTK 4 tried to strike a balance of sorts between 2 and 3, bringing the game's pace up without sacrificing too much detail in the process. Speaking as objectively as I can, RTK 4 is probably the best one as a complete package; diplomacy is deeper, the general pace of the game keeps things engaging, siege weapons and multi-stage battles add further variety, and the AI is smarter and less passive. RTK 5 never got an English release.
Of the newer RTKs, I've only played 6, 8, and 10. 6 is disappointing with fairly plain city building and arguable the worst combat in the series. RTK 8 is my personal favorite. It has some interesting RPG elements and the ability to play as an officer instead of just a ruler, but it's the amazing combat system that really does it for me. It's big downside is that the RPG sections you do between battles and strategy meetings start to get increasingly monotonous after a while. RTK 10 isn't a bad game, it's just not really my thing. There's more RPG elements and battles have more detail, but the combat system isn't as fun and it felt like the series was starting to drift too far from its roots.
The first Dynasty Warriors game was a Soul Calibur style weapon based fighting game. In Japan, the hack-and-slash games started out as a spinoff of that fighting game, but for the North American releases, they made them all one continuous series.
