As a former fan of the musou games, I'd say Samurai Warriors 2 is the best place to start. It's solid, it's got content, it's got musashi miyamoto, it's on PC. SW3 is better, but it's a wii exclusive so you gotta emulate it. Warriors Orochi 1 is appealing for being a crossover, but it's also broken as shit and you're going to lose because your incompetent "leader" is going to get his ass kicked no matter what you do.
Now, you might argue Dynasty Warriors is the father series to Samurai Warriors and therefore you should play it first, but Dynasty Warriors Extreme Legends Special Edition is pretty much the ultimate incarnation of that series. There is very little point in playing any game prior to that, but for information's sake, be aware that Dynasty Warriors 1 is actually a fighting game, and the actual musou series started with DW2, which I had a blast with on the PS2. Also, that DW6 fucked with the combat system and was reviled as a result, and that DW9 did even worse. But don't touch anything from SW4 onward.