I'm also a little bit of a (left-handed) pen geek, and I've used all three of these, plus the Hi-Tec-C. The Hi-Tec-C lays down a very nice, smooth line, but it's probably my least favorite because the narrow plastic barrel gets uncomfortable after a few minutes.
Out of the other three, the Zebra Sarasa Clip is comfortable to hold, but I don't think it writes quite as smoothly. The Signo and G-2 are both comfortable and smooth-writing... I've tended to favor the G-2 since I've been using some variant of it since high school (nearly 20 years now), and the 0.38 is even nicer. I'll probably end up picking up a few additional Signos though -- they write nearly as smoothly for me and I really like the blue-black color that the G-2 0.38 isn't available in.
I find the G-2 is the smoothest overall for me, but this may also be because of the relatively cheap writing pads I use at the office.
