After getting into them a while back these are the ones I'd recommend the most.
Samurai Warriors 2. Really awesome game, the set of characters they have is pretty decent too.
Dynasty Warriors 5. Really big selection of characters, and the Empires mode is one of the best.
Samurai Warriors 4. I like the gameplay of having two separate characters on the board, it encourages strategic placement and thought about who should take on what situation, also has a big roster. SW4-2 is a bit rough but I'm still going through that one.
Warriors Orochi 3 Ultimate. One of the biggest rosters in the series, and probably the best out of the WO side series.
Dynasty Warriors 8. the Lu Bu campaign is worth it alone.
One Piece Pirate Warriors 2. Really captures the style of the series well, and it has some puzzles and such in the world so it's not all mindless button spam.
Really, there isn't too much difference with the series outside of a few gameplay quirks that are exclusive to each subseries, like how starting from SW4 you have these dash attacks that let you move while clearing out peons, or how you have your switchable weapons in some of the Dynasty Warriors games. The biggest differences you'll see is between the Empires games, every one of them have different mechanics. Hope you like them, they're pretty fun to just load up and whack away for a couple hours.