my absolute favorite is the pilot hi-tec-C. they're not really available many places in the US, but you can order them from jetpens.com or from many sellers on e-bay. also if you can find random japanese markets nearby, you can get them there. the pen just writes so smoothly, it's available in 0.25, 0.3, 0.4, and 0.5 in about a million colors, and the new ones even have grips!
a close second for me is the uniball signo. it's another japanese pen that you can't find very often here in the US. these ones come in as small as 0.18 mm! that's a little too small, though...tends to scratch the paper a lot. their 0.28 size is very smooth.
i'm excited to know that staples is now selling the pilot g2 0.38 mm though. i may have to pick up some of those! 🙂