I like the brush tamiya includes with their panel liner. The bottle is steady as well and not prone to tip over.

I opt for dark gray than black, as it does not over power the overall look.

I love this stuff for popping out details on ships, and for panel lines on lighter colored planes. Works great on bare plastic, gloss undercoat, or flat paint if you do a clearcoat of future or something first. If applied directly on flat paint, it will look bad and seep into the paint instead of following panel lines.