Here, I must point out - it depends on the moleskine too. For my Moleskine Sketchbook, I have a perfect pen. Pentel Energel 0.7. It works fabulously on thick sketchbook pages.
But my quest this time was for plain or ruled moleskine, which has thinner pages. After reading several reviews from other moleskine users, I bought five pens.
Uniball Signo 207, 0.7
Pilot G-Tec - C4
Yes, four of them were Pilot, because it's a good, trusty brand, and I knew it was just a matter of finding the right fit. I tested all of them, and these are my results:
Pilot G-Tec-C4 and Pilot G2-0.38, while excellent are not for me. Their very fine point are more suited to sketching than writing. For writing, I find them bit rough on the page, and the writing is quite light. But I tried a few doodles, and it definitely works. Unfortunately, I am not much of a sketcher, so I will just find some other uses for them, or there is always doodling.
Pilot G2 - 0.5 - very good. This is the pen most frequently recommended by other moleskine users. I liked it. It's comfy, and it flows, but I did find it bit hesitant on the page.
Pilot G2 - 0.7 and Uniball Signo 207, 0.7 - my favourites, and perfect for me. I don't know if I like one of them better. I guess I will find out after more use. But the ink flows very smoothly. There is absolutely no roughness to it. Writing comes out properly black, dark, just as I like it, and 0.7 is definitely more suited to A LOT of writing. Between the two, I would buy either one of these or both for use with my moleskine, and probably other writing material too.