Ok.. I've nearly finished the chain I have been working on, and can't decided on the next:

Trellith chain, for ability - as my last chain was the Mylaria one, so to push my production up further.
Limitless travel chain (mission), short mission, Gives 4 35 energy artys. I'm energy (not rank) obsessed.
Prototype engineering chain - Of mid length, for the mining planet structure at the end.

advice?

your ank and energy would give us a nice idea of what to suggest

Rank 201, Energy 1310 (With relays on - 1680 without)

Dont do things that focus on gaining you more energy.
Instead, focus on doing things worthy OF your energy.

Trellith chain is a great example. It grants powerful abilities, and will be a benefit to you forever. This is what i would recommend the most. THe whole chain gives great stuff, and the final ability is just flat out awesome.

150 more energy is nice and all... but you will get 150 energy forever at the cost of 25, 800 energy. That means it will pay itself off in 172 refills. that isnt THAT much.. but i mean, you could be working towards trellith!

Or, for the same energy as limitless chain you can do The Aeon Moon and get 3 fluxes, as well as get up to the final mission in that chain for unlimited temporal fluxes.

or you could get 5% of the taltherian chain done, once you hit rank 230

GL- endless choices. All fun.

Trellith is the best out of the three I'd say.

taltherian chain will help you out in the long run