Most optimal Legion?

The best way to play them would be to play a better killer.

But to be serious for a second: Their playstyle honestly hasn't changed that much, you still want to try and hit as many people with Deep Wounds as possible and then try to chase one person down. The main difference is that you can no longer use your power to do that, you'll have to chase them "normally" now. A lot of tech or exploits depending on how you viewed it all are also no longer possible, you can't spam lunge anymore to gain extra speed neither can you moon walk or cheese the Deep Wound timer in any way(in case you were into that).

Regarding optimal builds and what not, this is all highly up to discussion IMO and there is no right answer here, that being said however: Their best add-ons IMO now are: duration(Mishief List, Mural Sketch, Stolen Sketch Book), recharge time(Scratched Ruler and Etched Ruler), Mend Time(Nasty Blade and Filthy Blade) and just the Iridescent Button. The rest is not really worthwhile.

Pretty good perks for them would be things like: Save The Best For Last(Essentially core, Feral Frenzy allows you stack this up fairly quickly and it gives you something to fall back on in those rounds where you won't get to use your power for anything but stacking it up), Bamboozle(Helps counteract the horrid vault nerfs), Thanatophobia(Usually not that good but in those games where you can keep everyone consistently injured it does wonders), Sloppy Butcher(Kinda the same deal as with Thana except people need to actually heal for it to do anything for you, they usually do though).

Despite those any other perks you would run on killers without any decent chase tools are pretty good, so Brutal Strength or Spirit Fury + Enduring for instance. Of course the basic meta perks are also good like BBQ, Ruin and Pop Goes The Weasle for example.