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New-old tenno, what's most valuable to hit next?
(dormi.zone)
I just came back from a long hiatus a month or so back, and also quit around the same time you did, so I feel your pain. This might not be 100% accurate, but this is how I see the various open world grinds from my own experience:
EARTH:
OSTRON: Obsolete for the most part, unless you really want a Zaw. Grinding this will give you baits to get fish that some later recipes need, but for the most part you're here for parts for a modular melee weapon called a Zaw. These used to be top of the line, but now are just pretty good.
QUILLS: Mostly obsolete. Not sure how far along you are in the quests, but The War Within will net you a new equipment slot for a modular weapon called an Amp. The default amp is hilarious garbage, and makes certain quests paaaaaainful to complete. However, the first rank of the Quills will net you the parts to make an amp good enough to deal with end game content so build that ASAP. Beyond that first rank though, a later Syndicate will give you better amp parts.
VENUS:
SOLARIS UNITED: Gives you parts to make modular primary and secondary weapons called Kitguns, as well as modular Moa companions. Neither is top of the line anymore, but kitguns at least are still a solid A tier in endgame content. You also need to grind this to max to open up Vox Solaris, so might as well!
VOX SOLARIS: Better amp parts. Still haven't started this grind yet myself, but that's basically it.
DEIMOS:
ENTRATI: Gives you more Kitgun parts, infested companions, and some good endgame weapons. Vulpaphylas (Infested kavats) are currently the meta, as their special ability is to self resurrect infinitely. Also gives you access to the Helminth halfway through, which allows you grind up old warframes into upgrade materials. Hope you haven't deleted too many to save slots! You'll naturally also grind rep while grinding for your first Necromech, which you'll need to unlock the last chunk of the game.
NECROLOID: Alternative necromech parts and guns. The default setup is really good, so I haven't felt the need to grind this at all, but you do you.
DUVIRI: High end melee weapons, and Incarnon Adapters, which are upgrade packs for older, powercrept guns (Soma Prime, my beloved...). Incarnon-upgraded weapons are currently top tier, but to use them you need access to the final locked solar system zone, which is again locked behind having a Necromech.
[ZONE NAME REDACTED FOR SPOILERS]
HOLDFASTS: Top tier weapons and arcanes. All but mandatory once you have access, but very quick to grind.