Easiest to use is Cosmic v83 by far, completey plug-in-play with very few left to fix or do if you're just playing locally. Bosses don't work that well though, they can DC you. There is a work around that, by changing the game's graphics to lower in the middle of the fight and maybe doing that again after like 5 mins or so, it's a bit of a pain having to change an in-game option when you're trying to survive the fight. Other than that though, it's really good and complete.
But I also recommend Tera/Lidium v111, it also works really well, you won't get headaches or creepy bugs except a bit of slowdowns when entering new portals and some other things that u might find weird but works, but in this one you should find fixes to some things scattered around or download from me, I'm slowly fixing the stuff left by finding fixes around. It's a WIP on quests, you can quest normally, random quests like "go there, get that, go there, kill 100 mobs" work all the time but the more sophisticated quests don't so you can't finish some important questlines, but if you don't care about questing and just want to kill some mobs and upgrade gear, v111 is really good. I haven't tested bosses out yet, but I've been playing it (a lot) in my free time, I'm having a blast, even though I haven't fought a single well-known boss so far. I fought Xerxes and it works, if he counts as a boss, he's got unique skills, the HP bar, he drops unique items and hey, it's a cool fight. ;p
Swordie v176 is great too if you're looking for something even more modern, it's the pre-5th job almost like latest update before 5th job and it's got almost everything there is to know about modern maple, but lots of things don't work and lots of NPCs aren't coded.