You could check out my review describing my "conversion" to HHKB:
https://geekhack.org/index.php?topic=48804.0Since that time, my favorite board is still the HHKB, but modified by installing Silencing Rings and lubing the stabilizers and switch rails with SuperLube 51010 oil.
My next-favorite boards are the RF87UB55, RF87UB45, V60MTS-C, and IBM XT (Model F).
The RF boards sound and feel more solid than the HHKB. The RFs have plate-mounted switches, whereas the HHKB has case-mounted switches. The RF seems more refined, but the HHKB is more fun.
I've also modded an HHKB by installing 55g domes, but I tend to prefer the standard 45g domes.
Overall, I prefer the 60% form factor to larger boards, and I prefer the HHKB layout to any other. When I use other boards, I remap them to something as close as possible to the HHKB layout.
Another nice thing about either the HHKB or the RF is the keycaps. Not only do they look and feel good, but they are highly functional. They are designed to form a unified functional unit with the switch. This is quite different with Cherry mx switches, for example, where different keycap sets can make a major difference in the sound and feel of the board because of differences in thickness, weight, and dimensions -- such as the relative length of the stem and skirt of the cap.
Having said all this, it comes down to your preferences. You will have to check out the RF and HHKB boards for yourself.
You can buy RF and HHKB from Elitekeyboards.com. They can also be found on Amazon and eBay, and sometimes through keyboard forums such as Geekhack or Deskthority.