I use default hard 4px round brush in Photoshop for inking at 300% original size. Once it's resized down to original size, it's very crisp :3
Personally, I find that 4px is the most comfortable for inking, since Photoshop's brushes are all really soft and their flow isn't the same as some other programs' brushes: 3px is too soft and the flow is not high enough, even at 100% Flow, and 5px is incredibly hard to control.
*If you have a heavy hand, adjust your tablet's settings to decrease pressure sensitivity; if you press hard and don't have a steady hand, the lines come out scruffy-looking + require a lot of erasing to clean up.
I have an extremely steady hand, but that only comes in useful when I'm inking slowly (eg. intricate shapes. More often than not, I actually just use Pen Tool for those shapes rather than inking by hand. In case you don't use Pen Tool - it's almost exactly like Bezier Tool in Corel programs and MSP.
Here's a tutorial for it.) So... having a steady hand only really comes in to inking if you prefer to ink slowly.
Faster inking results in smoother lines; I don't trace my sketch precisely, but rather 'redraw' the whole thing, using the sketch as a guide. If I try to 'trace' my sketch exactly, the result looks very scruffy and somehow artificial. My sketches are really clean, and I sometimes actually just clean up the sketch to use as lineart instead of inking it >w<
Inking with a sketching motion may also result in the lineart looking scruffy; have you tried one-stroke inking?
Recommended programs for inking:
If you don't feel like resizing / prefer to keep one working size, try SAI and openCanvas. I use them to ink different sorts of pieces - SAI is good for elegant lines, oC is good for action pieces, and Photoshop is good for bold lines.