In NO particular order:
Prophet Rev2
Value: The value here is off the charts. If you are only looking to spend $1,300-2000 for an analog poly, I feel its a no-brainer, get this before Korg Prologue or competitors. The 5-octave key bed feel is perfect, the weight and after-touch response are second to none.
Versatility: The range of sound possibilities is incredible. There is a bit of menu involvement when it comes to tapping into all the modulation possibilities, but it is still very accessible and the user interface well-designed.. Nothing anyone could call menu diving.
Strengths: Polyphony and tuning range really pays off in strings, pads, patches with long release times, complex evolving sounds, etc. DCOs tend to retain their tuning better across multiple octaves and because they do not stray until told to, often tend to be easier to work into a mix. Amazing modulation options!
Weakness: Bass is good but not on a level with it's VCO siblings, and the same can be said for the Curtis filter. Only one FX per layer should be at least two.
Subjective Comments: Spending a little more for 16 voices is worth it. You can add to the 8-voice version later, but it's more cost effective to go 16-voice early on. Having dual-timbral layers with 8 voices each is a treat!
OB-6
Value: It is the most expensive of the bunch, and I feel that is no coincidence since it is the result of a partnership between Sequential and Tom Oberheim, and that usually means an extra royalty or two must go to the partner.. and why not? Tom's a legend! The parnership between the two is magical. I only point this out because the higher price tag does not make the synth "better" or more premium than its sibling the Prophet-6, it's just that licensing and what-not add a couple of hundred dollars to the price over the Prophet-6, when the parts that go into it probably cost a little less to manufacture (for example, the lighter feeling keys, the thinner wood panels, etc.). But anyone who owns an OB-6 will tell you they are not bothered! It has that Oberheim state-variable filter, and it sounds fantastic! You already know if you want that Oberheim sound, and if you do the cost proposition of a new OB-6 versus vintage options a great value indeed.
Versatility: The OB-6 is a versatile poly synth, just perhaps not as versatile as the other synths mentioned here. The 12db state-variable filter results in a certain character, and that character shines through across much of the sound palatte. This is a good thing! But if it is your only poly, as a desert-island synth, it might not be the only synth you ever need.
Strengths: Classic sound, the filter can get so lovingly growly and dirty sounding. Great low end, amazing bass sounds, pretty much anything you want from a 6-voice poly. Certain synth sounds I just can't get anywhere else. The same "hands-on" playability we find on the Prophet 6, which makes it so easy to find a great sound quickly, even starting from a basic patch.
Weakness: Lots of character in specific areas makes it less versatile than the others; at times harder to work into a mix. 12db filter can make it sound less "dynamic" than the range and characteristics of 24db filters. Modulation options very limited compared to Rev2, but equal to the mighty Prophet 6. The overall richness of the sound can become a liability in certain situations.
Last but absolutely not least!....
The one and only Prophet 6
Value: At $2,800 USD for the keyboard model I feel this is the sweet spot for a premium 6-voice analog poly, and for what you get, the price is right. If this feels expensive, keep in mind that when the original Prophet 5 was introduced (1978), it cost around $4,000. Adjusted for inflation, that would be about $16,000 USD today. The Prophet 6 doesn't sound exactly like a Prophet 5, but different is not lesser...
Versatility: More versatile than the sound of the OB-6's filter, but less versatile than the Rev2's modulation capabilities. The Prophet 6 again hits a sweet spot that may classify it as the "best all around" of the three synths mentioned here. The filter initially seems to have a bit less character than that of the OB-6, but when working with it musically it often feels like it can just "do more". The end result of sound design with the Prophet-6 somehow feels greater than the sum of its parts (and subjectively, perhaps greater than the other two synths mentioned here in terms of overall sound design range).
Strengths: I'm a stickler for good bass and the P6 is the best synth I've ever owned for that purpose. I feel it destroys everything else, including Moogs. OB-6 can hang with it, but P6 does it slightly better. For other sounds, the filter which some describe as "thin" compared to the OB-6 actually becomes one of it's most formidable weapons... The preciseness of the filter allows it to create sounds which carve themselves out in a more defined way compared to the OB-6, and that pays dividends in a mix.
Weakness: The list of weaknesses grows short for the Prophet 6. Yes $2,800 is a lot of cash, but not compared to vintage synths of the same caliber, and not compared to OB-6. You could say the keyboard needs another octave (as does the OB-6), but since it's a VCO synth the overall tuning range is not going to be like a DCO synth. This is an odd comment, but I keep my studio low-light when making music, and sometimes the labels on the synth are hard to read, where they are clearly visible on the OB-6. Like most, would love more LFOs.
This is somewhat nitpicky, but its a general weakness for those who own both P6 and OB-6 - Sometimes I wish the knobs and various functions were placed in the same location on both OB-6 and P6. The Rev2 feels like a different beast entirely so it's not as much of an issue, but it often feels weird to go all the way to the left to reach the volume knob on one, then all the way to the right for volume on another. Or maybe I reach to change the unison key mode and the buttons are in opposite locations. It would smoothen my workflow if general placement choices were similar between the two, but I have a feeling the design choices were made based on vintage synths, which I fully understand.
Overall... I regard all three synths as three separate instruments, each with their own pros and cons. One is not better than the other.