I needed new V-neck T-shirts and had no idea which brand to buy.  So, as an experiment, I ordered some Fruit of the Loom (abbreviated FOTL below), some Jockey (J), and some Calvin Klein (CK) and compared them.  Please note I'm not an expert on garments or fabrics, this is just what I observed among my samples.  My shirt size is Large Tall--I'm 6-4 and weigh about 170, if this helps.  All three shirts fit me fine, although not exactly the same.

Chest circumference.  FOTL and J are slightly smaller (about 40 inches) than CK (measures 42 inches).
Back length.  FOTL 28, J 29, CK 30 inches.
Fabric.  None is what I'd call heavy or beefy. FOTL fabric seems thinner and rougher than the others.  J is slightly heavier--possibly the heaviest fabric of the three--and softer and smoother than FOTL.  CK even smoother--I'd say that of the three, the CK fabric is the most comfortable for me personally.
Features.  All three have a comfortable, taped neck seam.  J and CK also have taped seams across the shoulders.  However, FOTL has an untaped shoulder seam.
Neck opening.  This is important, because of course the idea of a V-neck is for the T-shirt not to show.  However, the neck openings of outer shirts vary, so the larger the opening of the T-shirt, the more accommodating it will be.  FOTL appears to have the largest neck opening of the three, CK is next, J is the smallest, at least among the samples I have.  Regardless, these openings are not deep or wide enough to accommodate some of my dress shirts.  This was the only real let-down among these V-necks.  Your mileage may vary!

I suppose the bottom line is that these three T-shirts are all competent, but the differences among them may cause one or another to be more compatible with personal preferences.  My own preference is CK, but--other than finding the neck opening too small for a given outer shirt--I don't have a problem wearing any of them.