Symphones V9: https://www.symphones.com/
Nhoord Audio RED: http://www.nhoord.com/products
Elleven Acoustica (various models): http://www.ellevenacoustica.com/products
Turbulent Labs X Driver: https://turbulentlabs.com/shop/turbulent-x-driver/
An OEM Driver from earphonediylabs.com: https://www.earphonediylabs.com/product-category/headphone-drivers/
These seem to be some of the primary choices I've come across in my research. I've found some other lists, but most of them seem to be 5 or more years old, and there have been a lot more and newer options since then. Currently, I'm leaning towards the Symphones V9, based on some of the very positive forum posts I've seen about them over at Head-Fi (https://www.head-fi.org/threads/grado-modders-go-magnum.576717/page-319), the success of their previous models, the elastomeric seal around the edge, and the fact that I think they look freaking cool.
Whichever driver I choose, I'll probably swap them into a pair of Beyerdynamic DT 990 Pro's, since you can get a brand new set pretty affordably (they're currently $115 on Amazon), Beyer seems to be known for their durability and modability, they have readily available replacement parts, and most importantly they use 45mm drivers, only 1mm off from the diameter of most of these drivers. As beautiful as I think many custom Grados are, I don't think I can get behind the on-ear design and use of foam pads.
Anyways, as far as what I mean by "best", I guess I mean the highest performance per dollar spent. According to some posters on the head-fi forum, the Symphones V9 matches or outperforms many more expensive headphones, which would be awesome if that's true. I'm not terribly picky about the sound signature, as long as the highs aren't too fatiguing. I would prefer something with a wide soundstage, since I don't have any open-back headphones, and I don't currently have any headphones that excel at soundstage.
If anyone has had experience with one (or more!) of these drivers, I would love to hear about it! All opinions and comments are welcome.