Um... It depends?
Let me approach this with skills.
Mining skill is only good for mining lasers. While I do a lot of Federation of Mining missions, that is more interest specific.
Weapon skills (and only pick one) are good for both combat and salvage. If you're not into combat, you have to wait for level 25 to clean the space lanes of other people's junk. I also really enjoy salvage, but I'm hesitant to invest in the Energy skill for the level three salvage laser.
Shields: No. If you are into combat, pick one, but otherwise, no. I'm level 47 and I've never invested the points to equip the one shield I made.
Scan Tech: This is a good one, as scanners make everything possible. You need scanners for combat, salvage, mining, and derelict exploration.
Device Tech: Cargo expander. Cargo expander, cargo expander, cargo expander, cargo expander, cargo expander, cargo expander, cargo expander. Device tech also powers some really useful devices, but the big one is your cargo expander. If you are primarily a hauler, you will live by your cargo space. (And, look up Quantum Space in the CEDR.) If you mine or salvage, cargo space is very valuable. For combat or derelict exploration, you do not use as much, but it's still really nice.
Reactors power everything. Unless you will be spending most of the game puttering around extractors and factories (nothing wrong with that, but no XP), you will end up needing power for everything. If you mine, you will be constantly looking for better reactors. Salvage equipment or a good weapon and shield can also eat up your power reserves.
Survey: There are usually GBM contracts for water and oxygen, and iron and copper are not uncommon. Water and oxygen have pretty low survey skill requirements, and so a few personal extractors are good for extra cash.
Drone: Even if you don't craft, GBM contracts for ore are not uncommon, and refined ore has a nice price. Five Hermes drones can get you a bit of extra spending money.
As for the actual equipment:
From what I experienced at low levels, the Storm reactors are the best, preferably +P3 for the extra power. Talk to your guild crafter, or check the emporium. Even without the power boost, storm reactors are good. When you can get a Tesla 1, do so if you mine, salvage, or haul. If you are into combat, a Hammer 1 & 2 may be better for the damage boost.
The QE cargo expanders are the best at their level, again preferably with +P3. (I would also say that many crafters will dissuade you from using three green crystals on a QE1, as you outgrow it so quickly.)
If you are into mining, which is a power hog, an Energy Injector or two may help. When you get to a higher level, the Babylon POD is a bit of magic.
If you are into combat, a Messiah Turrent POD is a good investment, as is a Taser Shield POD and Midas POD.
If you are into Derelict exploration, a small grav sled is probably your most important investment.
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