Dynasty Warriors 3. It was the peak too early, all they had to do was keep making more just like it with slight improvements and they tried with DW4 and DW5. But then they actively made it worse with each entry, with the pit being DW6. It has been a slight incline ever since, but they still have yet to come even close to the peak again.
I haven't played every version, but DW3 is my favorite partially because I liked how they handled the ultimate weapons in that game.
Getting the death element on Lu Xun's special attack was OP, but at that point in the game, who cares? It's just fun.
I just revealed it as my #5 game, I speak a lot about it and Wu was certainly my favorite. Gan Ning's ultimate weapon was just straight fun too with the extra lightning effects.