I'm a big fan of particular writing instruments. My preference is as much about feel and heft as it is about the way it writes and the way it looks.
My favorite fine-point pen is the Rotring Rapidoliner (disposable) 0.25 mm (white tip). This gives an extremely fine line. Not the finest, though -- there is a 0.18 mm Rapidoler, which I have not used. I used to use refillable Rapidographs for drawing: my experience cleaning them and cleaning ink stains from my fingers has led me to greatly appreciate these disposable pens.
This pen has been discontinued, unfortunately. I have a few refills on hand, since when they started disappearing from store shelves I went scrounging for them and stocked up, but it seems I won't be using them for too much longer.
There are some downsides: it is "scritchy" because the tip is so thin, and requires smooth paper for best results; it clogs easily, and if you've left it alone for a couple of days you will probably have to spend some time shaking and coaxing it into delivering a line; it is somewhat expensive; and it seems wasteful to dispose of both the tip and the ink supply while keeping just the pen barrel. Also, the ink tends to bleed through thin paper like that in my Moleskines.
My favorite mechanical pencil is the Rotring 600 matte silver model with the knurled grip. This is also discontinued, and they are now going on eBay for $50 or more. There are later versions that have a smooth finger-grip area, but that doesn't seem right.
The old-style 600 is heavy, made of brass, nicely balanced in the hand, and seems nearly indestructible (it had better be, because probably won't be able to buy a replacement). I have this pencil in 0.5 mm and 0.35 mm tips. I found the 0.35 mm pencil on eBay from a seller in Germany. The 0.35 mm lead is not widely stocked, so I had to order some. I find that I like the fine line, but it feels like I am cutting the paper with this very thin lead, and because it is so thin, it disappears very quickly, so I am constantly having to advance the lead. I wind up using the 0.5 mm pencil most of the time. Both are a pain to carry around because unless they are in some kind of case, they will snag and tear holes in your clothes or scratch up anything else they are rolling around with.
My notebook of choice is the black Moleskine, in pocket size with blank pages for personal use, and the slightly larger 5x8" size, with grid pages, for work. I also like the brown kraft Cahier notebooks, also with grid pages, for my hobby programming projects.
My eraser of choice is the Pentel white Hi-Polymer eraser. This is particularly nice for Sudoku puzzles because it is soft and can be used repeatedly without tearing holes in the paper. This means I have to carry a loose eraser with me. I'm keeping an eye out for some kind of idea case that would hold pen, pencil, supplies, and one of these erasers, but I have not found it yet.
It seems like there is some improvement taking place in mechanical pencils. Pentel has models that acknowledge that a lot of space in the mechanical pencil is wasted and could be used for a larger eraser (the tiny pink or blue erasers in the traditional mechanical pencil is nearly useless). They let you twist the barrel to extend the eraser -- you don't have to remove, and possibly lose, a metal cap. This falls nicely into the "why didn't someone think of this before?" category.
I'd pay quite a bit for a pencil with the lovely finish, heavy brass body, and knurled grip of the old-style Rotring 600, but which uses a larger and longer white polymer eraser, like Pentel's, that I can extend with a twist. It should also have a retractable tip so I can carry it in a pocket.
I would also bend over backwards to give a manufacturer money for a slightly less expensive Rapidoliner that had a bit more ink in it and that was readily available in 0.25 and 0.18 mm.
The other pen I love is the plain black PaperMate Flair felt-tip. I can't think of a single design improvement I would make to that pen, but if I could, I would figure out how to make it with waterproof ink. It seems to bleed through my Moleskine paper less than the Rapidoliner. This pen has been essentially unchanged for my entire life; I think it was introduced in 1966!