1. Buff / Debuff stacking works like this:
You can only have 1 Damage Buff and 1 Debuff up at a time, so what does this mean? It means that even though WoR is active (which you see on the side) if you already have WoL (which is a 30% buff while WoR is only 25%) you are still taking the healing effect of WoR, but the damage buff is coming from WoL. At the same time, you can only have 1 30% Debuff up at all times, this being something like oppressive darkness, melting point, or shattering strike. That's basically all there is to those.
2. Highest DPS:
Currently the "highest" DPS is Whisper of the Worm, but not in short bursts or when certain weapon combos are necessary. For most boss fights Izinagi is actually the most powerful DPS, and is also able to pair with weapons such as Recluse and Wendigo GL3. Currently not much can touch these weapons besides something like an Anarchy and a Firing Line sniper rifle, but overall the best loadout for DPS and PvE in general right this moment is Izinagi, Recluse, Wendigo.
3. Fastest rank ups:
The method that's worked best for me personally is to complete every planetary bounty (besides Eris / the Moon) on all 3 characters daily. I've been seeing a massive influx of EXP every day and have been climbing ranks since.
I hope this all helps and let me know if there are any more questions!
Awesome! Thanks for the info! So the buffs and debuffs essentially take the "best" of that type? So since WoL has a stronger damage buff, its damage is given instead of WoR, but WoR isn't completely nullified?
Yeah so the effects of WoR aren't completely nullified, the game is just taking the higher buff like you said. I believe all Debuffs are currently a flat 30% so no matter which one you have it'll work, but some are easier to use than others.