The OP asked about syn gut, not polys.
Incorrect. Polyester is a synthetic string material, too.
The best synthetic strings:
1.
Gut-like playability (dynamic stiffness):
Most likely Ashaway Dynamite Soft 18, followed by the 17
However, with the data available: Ashaway Dynamite 17
TW only tested the very thick 1.41mm Dynamite WB 16 and it still was closer to gut's dynamic stiffness than other synthetics.
Compare for stiffness, for instance:
Ashaway Dynamite WB 16, strung at 51 lbs, 1.41mm,
109.7
Pacific Tough Gut 16, strung at 51 lbs, 1.39mm,
110.3
Now, the Dynamite loses more tension than the Pacific Gut, which explains why it beats it in softness, but it's still the closest competitor to gut. The new Dynamite Soft uses thinner filaments of Zyex so it should be even closer.
RSI's numbers are slightly different, putting the following strings close together (62 lbs fast swing):
Head Perfect Power 17, Nylon, 1.22, 144, 14.8, lbs lost
Gamma Professional 18, Nylon/Zyex, 1.22, 144, 15.15, lbs lost
Alpha Gut 2000 18, Nylon, 1.21, 146, 14.67, lbs lost
Ashaway Dynamite 17 , Zyex/Nylon, 1.24, 147, 13.72 lbs lost
Dynamite exhibits its gut-like dynamic stiffness more and more as stringing tension is reduced, so it's not super surprising that its high tension numbers from RSI are about the same as the softest nylon multis. Keep in mind that Dynamite tends to be even thicker than its reference gauge, so it still is likely the softest among these strings at high tension with a fast swing.
Unfortunately, every one of these strings is missing from the TW Database.
Worst: Prince Problend w/Duraflex 16 (Main). Or, if you don't want to count hybrids: Gosen Arammix Pro 16.
2.
Tension retention:
62 lbs fast swing (pre-stretched): Gamma Synthetic Gut 16, 3.3 lbs lost
pre-stretched 62 lbs medium swing (pre-stretched): Prince Topspin Duraflex 15L, 3.1 lbs lost
pre-stretched 62 lbs slow swing (pre-stretched): Prince Topspin Duraflex 15L, 2.6 lbs lost
62 lbs fast swing (no pre-stretch)(no pre-stretch): IsoSpeed Professional Classic 17 and X-One Biphase 16, 9.0 lbs lost
62 lbs medium swing (no pre-stretch): X-One Biphase 17, 8.8 lbs lost
62 lbs slow swing (no pre-stretch): X-One Biphase 16, 8.4 lbs lost
51 lbs fast: X-One Biphase 17, 7.7 lbs lost
51 lbs medium: X-One Biphase 17, 7.5 lbs lost
51 lbs slow: X-One Biphase 17, 7.1 lbs lost
40 lbs fast: X-One Biphase 17, 6.2 lbs lost
40 lbs medium: X-One Biphase 17, 6.0 lbs lost
40 lbs slow: X-One Biphase 17, 5.7 lbs lost
Note that I don't think TW pre-stretched every string they tested at high tension. I don't think Dynamite was pre-stretched, for instance.
Worst (RSI): Ashaway (braided) Kevlar 17, 1.22mm, 28.37 lbs lost
Note that there are no TW numbers of Kevlar strings – who knows why.