reMarkable's Markers are what are best tuned to their devices. If you're looking for a best anything for the reMarkable, it could only be something that they themselves produced.
That said, the pencil's implementation in software is close to realistic but not perfect. If rM's Markers don't give you what you're looking for, it's hardly likely any third party hardware, whether stylus or nibs, will be any better.
I don't fully agree. Certainly RM knows their hardware and software better than anyone and have the opportunity to dial in the pencil function better than anyone else; however, I can see why they might not opt to do so, preferring perhaps to create a stylus that is more "balanced" to perform well as calligraphy pen as well as a marker or a ballpoint as oppose to excel for the limited purpose of pencil sketching.
I love the Uni digital from Mitsubishi. It feels like a pencil. My only complaint is that it's a bit short, I would have liked it the length of a regular Mitsubishi uni new pencil. Some people have commented they find the size perfect. I have pretty big hands.
Also, I could only recommend it if you find it at a "normal" price. I've seen it $70-130 USD, which is insane. I got mine for $50, and that was with shipping. I think in Japan they go for $35.