So if a person only have a limited supply of GP, what are the best ones?
I think the completely agreed upon 1st choice is probably F.A.T.H.E.R, who won't want those?

But what after that? From assorted info I've gathered, it seems like the following are some of the best plans:
for a lower lvl player(150 or lower especially), 10x terraformer to get mission exotic
QSE in general, because it's one of the rarest object. And while there are racial ability to boost res(as well as terraformed being more readily available in drops) it's fairly easy to get a 15x mega at high lvl, and thus size is all that really matters in the long run
radiant surge at lvl 800+

it would be nice if a ranking of sorts is provided, something like this:

F.A.T.H.E.R
radiant surge cannon(lvl 800+)
Terraformer(lvl 150-)
QSE
Terraformer

Note that there are two terraformer in the ranking, because its value greatly diminishes once u can NPC like mad AND have access to racial abilitys such that u get equivalent of 10 use of it a week easily. Still a very good thing, but it does get to the point where it can go no more.

There going from most important to lest , in my opinion.
1) F.A.T.H.E.R
2) 12 Terraformer's to get a 15x mega mission exotic.
3) 10 QSE for the mission dyson.
4) radiant surge cannon(lvl 800+)
5) 10-30 QSE for rifts. (only use on 15x mega or better)
6) 5 Mineral Storage Tower , 5 Artifact Warehouse , 5 Research Data Archive. ( After getting all 5 mission planets you will most likely need more storage)
7) Planet Flux. (Its good to keep 1 or 2 on hand if a good planet gets attacked)
8) energy refill. (If your slow ranking or think your out growing your tech skip to next or wait for tech)
9) Durtanium Brackets and Android Helmsman. (This is more of a rich mans option and not recommended. There easy to get in game and it would take 100's of each , in the long run , to do anything)
10) Spin the Galaxy Wheel.(Your not looking for credits , your trying for rank points. Something you would only do if you plan to spend 1000's on this game)

FATHERs are the only thing I've ever bought for any online game.

If GP is limited then I'd NPC to get the terraformers. It'll take anythng between 4000-7000 NPCs to get enough but you'll get lots of other good things like Toxic Purifers, Planetary Data Optimisers and even the odd planet flux.

QSEs are really hard to find. It's worth saving your GP for these instead of terraformers though the hyperterraformed planet is really useful early on.

If you NPC you'll be able to boost your mining planets hopefully enough to afford better guns. This would allow you to hold off on getting Radiant Surge Cannons till costs to boost attack get extreme (e.g. QCB technology weapons) by which time you'll be much higher rank.

Of the rest of the artifacts the Planet Flux and warehouses are the most useful, especially at lower rank where caps are small. These are available on the forums. If you make sure you have high AP to get tradeable artifacts and NPC a lot to get CTP you can buy these as you go. Of course if you have GP to spare then they're great investments.

I'm rank 500+ and have over 10k production of every resource. I've manged to do that by NPCing my way to good planets.

FATHERS and Quasi Spatial Expanders. Everything else, like Remainder said, can be farmed from NPCs or traded for.

I tried to resist but failed.... Save a few GP to abandon a couple of the uber-crap planets everyone colonizes when they start playing.

i wouldnt get terra formers. once u rank and npc more, getting 12 for exotic is no sweat

Me, I have spent a lot on this game and have never even thought of buying them.

This would be my choice also. I would suggest 11 QSEs to make your 15x mission exotic very massive (up from Massive).