I'm a waterbased printer (rather than plastisol), but I don't think that changes the answer: black, hands down. The ink is saturated with pigment. Smooth, thin, runny, and forgiving. Never have to worry about contrast, and it always goes on smooth and simple. You'll never have a problem printing black, unless that problem is mess.
In general, dark ink on light fabric will be the easiest prints. Black is best to work with, but navy is also great, etc. Plastisol is less affected by colour consideration than waterbased ink (e.g. white ink on black fabric), but it's not immune.
In general, dark ink on light fabric will be the easiest prints. Black is best to work with, but navy is also great, etc. Plastisol is less affected by colour consideration than waterbased ink (e.g. white ink on black fabric), but it's not immune.