I probably know more writers who use the Pilot G2 than anything else.
But when I write fiction with a pen, I very much prefer a good fountain pen with a fine nib. One with a bladder, not those cheap things with the plastic refill cartriges. A good fountain pen writes without effort, and the nib soon forms itself to your unique style. I also use a fountain pen for journaling. Waterman makes some excellent fountain pens at very reasonable prices. So does Conklin. My two favorites are the Conklin Mark Twain Crescent Fountain Pen, and the Waterman Carene Matte. Writing with a good fountain pen is a pure joy.
Then again, like Shelby Foote, I sometimes use a dip pen for writing, and I have been known to cut my own goose feather quill and use it.
I also use mechanical pencils fairly often. I use the Sanford Ph. D. most often because it has a wonderful feel, but I'm also getting really fond of a very heavy Pierre Cardin mechanical pencil that takes 0.9mm lead.
When not using a mechanical pencil, I use Ticonderoga 2.5 woodcase pencils. The Ticonderoga is the traditional pencil for writers, and I like sharpening them. It gives me time to think. John Steinbeck recorded using 25 dozen Ticonderoga pencils in the writing of East of Eden.
But when using these, I also use a slide-on triangular grip.
Writers cramp can often be avoided, or at least greatly lessened, by using a writing stone. Traditionally, this is a smooth stone held in the hand behind the pen or pencil as you write. Cramp is often caused by the position of the ring finger and the little finger, and a writing stone keeps these fingers supported, and in a position where cramp is less a possibility. Using steel Chinese exercise balls for the hand also helps. The 1.5 inch ones are best.
You can also make a form fitting writing "stone" from Playdough.
The main cause of writer's cramp is not writing often enough, of course. The muscles aren't used to the activity, and cramp when used. But a writing stone, and a good, thick, heavy pen or pencil with good balance helps greatly.
Cheap pens and pencils also cause cramps. A good pen or pencil has some heft to it, and has a grip thick enough to hold without squeezing. But the thinner the pen, the more you cramp. The lighter the pen, the more you cramp.
You barely even have to hold a good pen or pencil. It does all the work through weight and balance. The weight makes it write without any pressure at all, and the balance means it stays in position without the need of controlling it with your hand.
Improper grip also causes writer's cramp, and just hold a pen or pencil correctly makes things much easier.