It really depends, it's mainly preference. There's a bunch of options, Badlion, Lunar, PvPLounge, Laby, 5zig, you name it. All of them have considerably similar hit reg, obviously mods that are already pre-installed in the said client are there to help you make the game run smoothly and just overall save time and not having to worry about the knitty-gritty like FPS boost for example. I guess if your a quote on quote "serious" pvper, 1.7 animations is what every person seeks for and obviously having good ping
Again, any client that is catered to primarily the pvp community usually does it job in doing that. So any mainstream client will do.
Also use one that is not ram heavy on your computer if you have a budget computer or laptop or anything else.