My go-tos:
Ballpoint: vintage Parker Jotters w/ Fischer Space Pen refills
Fountain pen: Parker "51" (vintage only)
Mechanical pencil: Pentel P205
I have others, but I like to keep my tools (whatever they may be) pared down to the essentials, and I could happily live with these three.
I used to like roller balls best, but once I started using fountains, I stopped using them entirely. I've never been a fan of gel pens. The line width and color is too perfectly uniform, and I guess that's part of why people like them, but I feel they have no character. The Pilot G-2 is a modern legend, but it does nothing for me and I don't even own one.
The Parker "51" is my favorite fountain pen. The hooded nib does a great job of keeping the pen from drying out too quickly when uncapped, making it practical for real-world use, and the fact that it doesn't look like a normal fountain pen allows me to use it on a daily basis in front of others without feeling ostentatious (which I would with a "traditional"-looking fountain pen). And most important, it writes well and is comfortable in my hand!
The Parker Jotter appeals to me in part because it's been made continuously since the 1960s. It was the first commercially successful ballpoint, and amazingly the same model (altered only slightly) is readily available today in any stationary store! Throw in a Fisher Space Pen refill, and I have something that fits in my pocket, is comfortable in my hand, and writes in as many conditions as you could expect from a pen. And it is as appropriate in a formal meeting as it is on a street corner. If I could have only one writing instrument, that would be it.
I have three "51"s and about half a dozen Jotters from all different eras. I won't be needing any more pens for a while!
