For Korg; Korg themselves have a bevy of their own VI recreations of their original hardware devices including the newer Wavestate, Wavestation, Opsix, Modwave, Polysix, and all the way back to the infamous M1 and Triton. Having owned, or still own many of the hardware devices, as well as owning the VI's, I can vouch that sonically (at least for recording) the VI's are virtually identical to the hardware. I love my hardware Korg Wavestate as it's currently my absolute favorite but I'd have to admit I can't hear a difference with my Wavestate VI. As I've mentioned here before I think most of these VI's are sonically identical to their hardware brethren but that's not to say the softsynths will ever, ever provide the tactile response and touch of hardware. No comparison. If however, you're looking for the tried and true Korg signature footprint in a VI, for me, there are no peers.
For Roland; Roland is now cloud-based/subscription-based and offers virtually all the icons of the Roland legacy. I subscribed briefly and as with the Korg VI’s I found the sonics as an exact duplicate. I love Roland sounds but with so much available for other unique sound signatures I choose to opt out. However, if I was a hard-core Roland fan and was looking for that specific sonic, I’d personally look no further.
Oberheim, Yamaha, Roland/Juno; For me, and if you're looking for nothing more than the original sonic sound, and not too worried about deep re-programming, sequencing, arpeggiating etc, nothing beats Arturia. Their OB-Xa, to my ears, brings back memories of my original OB. Although I never owned one I do have a friend that still has one in his collection and Arturia’s take on the endlessly fat Yamaha CS-80 is ridiculously close. The same goes for the Juno. Arturia’s Jun-6 V is an almost identical copy including the now-famous chorus. Also, I really like the software designer “Soundpaint” who is taking a different approach to VI development. Their Juno patches are really great, but to my ears, it’s a bit more of a Juno +. In the end, if you really want the Juno sound, Roland still has the best in their VI collection but personally, I’d be just fine with the Arturia.
DX7; I’m at a disadvantage here as I’ve not heard the Reface DX. I owned an original DX-7 so I’m really familiar with those sounds. Once again, and for me, the Arturia DX7 V is as close as I’ll ever need. Maybe close to identical. That said, the DX-7 at its heart is an “FM” based architecture so I’d guess the Reface DX, and I know the Arturia DX7 faithfully captures the spirit of the DX sounds but if one is into FM synthesis in general and particularly programming FM synths, the world is your oyster. There’s more out there by way of FM programs VI’s to last a lifetime.