What I usually use are a combination of Airbrush and Water tool . If I want rough stuff, I'll go for marker tool.
Alright, here's a more detailed shot on what I use for my brushes. I still had a lot of people asking me what settings i use for my stuff I will be putting this on my FAQ xD~! well, plus i'm using the marker brush a lot more now, so here are my settings : ). If you have any more questions, feel free to note me xD;, but i feel like i pretty much covered it all here .
I had a problem with the marker tool for a long time. no matter how much on top of each other or how hard I pressed, I wouldn't get anywhere close to the color I chose.
solution is simple though, it's the hard <-> soft settings in advanced settings mine was at 100, default for most brushes and usually don't affect the opacity/density (normally only affect that you won't have to press as hard to get thicker lines, of if your lines turn too thick, lower it to get thinner lines easier)
by increasing the hard <-> soft to 150-200 it worked normally ^ ^
advanced setting part of brushes can play a huge part in how the brush functions c:
just figured I'd mention it since the advanced settings aren't visible and those settings I struggled with was the default ones from SAI's site, that I downloaded and purchased it from ^ ^
there are brush sets you can download online for SAI, and there are tutorials that will teach you how to install those brushes . mine came with my SAI so i can't help much in this department. Good luck.