Eigenbarrel has the longest control bore, good honing, lots of porting area, highest theoretical efficiency (meaning lowest theoretical operating pressure), and is also the cheapest. I think that makes it a no-brainer.
My Shaft 4 on my Geo 2 works fine lol. I find myself in snake quite often, so a quiet and accurate barrel helps a lot. I find the stock Shaft 4 to be a great shooter and ask almost everyone I shoot out, as well as my teammates who are close by me, and they claim to not hear me over other noise on the field. Maybe they're saying that because they are too zoned-in and drown out other noise. I think it's pretty quiet, beautiful "mirror" honing, and has a good control bore. I've had no issues with it thus far.
Blood Fade Planet Eclipse Nexus Signature Series Director's Cut 1/2 aka "The Blood Nexus"