Currently have a bunch of parts to build out rifles with, wondering what would be the best combination for competition would be without having to buy additional parts.

I'll be building one out as a comp rifle and the other will be a standard rifle build.

I have the following parts available:

Uppers available: LWRC DI, Geissele Super Duty, Sig M400 (all 16")
BCG: LWRC or Geissele BCGs
Gas block: Superlative adjustable gas block, LWRC block, Geissele block.
Buffer tube: Carbine buffer tube, rifle buffer tube, and VLTOR A5 tube.
Springs: Standard carbine, standard rifle, Sprinco red, Sprinco Green
Buffers: Carbine, H, H2, H3, H4, A5H0(3.8oz), A5H1(4.5oz), A5H2 (6oz), standard rifle buffer, Armaspec Stealth H2

For those that have tinkered with buffers with an adjustable gas block, what have you found that works? Any difference?

Thank you all in advance

Normal carbine spring & H2 is my preferrence for everything.

Define "Competition" ... Long range precision, 3/2-gun, etc?

Depending on what comp you want to shoot I would just get something together you are comfortable with and shoot a few competitions and let the need for change drive the configuration.  Many and myself built what we believed is the ideal comp rifle and then we shoot a competitions and find out real quick that parts X, Y, and Z just did not work, were excessive unneeded weight, out right failed, and so on.  The things that made the biggest difference for me were losing weight, a good brake, predictable consistent trigger, and an accessible reload.  The first few options I tried for reload carriers worked but were not ideal or failed.  Also talk to the other people who shoot the competitions you plan to shoot and watch what is working and what people are having difficulties or failing equipment.

The one that makes the rifle run best (and we can't determine that from the info given),

You probably want it to go bang every time, yes? Start with standard weight everything, that's why its called "standard", then change one part at a time for a given ammo type, or whatever.

The AR is pretty well refined. The only time I deviate from “standard” buffers and springs is with non-typical configurations… i.e. .308, 9mm PCC, that did not function well with standard components.

Start with standard parts and go from there to address specific issues.

ETA: one trend noted when dealing with the .308 and 9mm is that more mass is generally  better than messing with specialized springs or gas blocks.

Also, more mass tended to widen the operating envelope with weak vs stout loads and suppressed vs unsuppressed