I have a 2015 GS350 Luxury trim retrofitted with 2019-20 coil springs, because the most luxurious 2016-18 coil springs are no longer in production.
Hence, my favorite in the range would have to be the 2016-18 GS350 Luxury trim; I wish I had been patient and waited another six months, then I would have had the 2016 midlife update.

The midlife update has more chrome front and rear for a more upmarket look.
A touch more chrome inside around the power window switches too.
The midlife update has more laser welding for a stiffer body structure, and this forces the same coil springs to compress more for a more compliant ride.
The 2016-18 also has the softest ride amongst the 8-model years.

The 2019-20 has intermediate sporting-luxury nee touring coil springs.
The 2015 has the firmest coil springs and also the firmest shock absorbers for the most controlled ride with tiring high frequency vertical oscillations - but it also handles almost like a compact sedan!
The 2014 coil springs are about the same as the 2015, but the 2014 shock absorbers are softer for slightly more float, for a lower frequency of vertical oscillation for more ride comfort.
The 2013 has different coil springs, although I haven't been able to test this vehicle back to back because it was a long time ago, but I remember the ride is more palatable.

The midlife update is fractionally smoother and fractionally quieter sounding, with fractionally superior fuel economy.
Update also has full screen sat nav, as well as instrument panel with small digital display.

 

What about the ES350 or ES300H in luxury trim or F sport for the digital speedometer? I'm only saying this because I used to own a ES350 and even though I drive GS350 now, sometimes when I go over certain familiar roads, I feel like the ES rode better over them than the GS.

Obviously, the GS has other benefits - I drive the crap out of the car and it definitely has more oomph and delivers wayyyy more excitement.

I also have the luxury trim so the seats are super nice but I don't have the digital speedometer.

 

My 2015 F Sport handles quite well in Sport+ mode.

 

If OP test drives ES350 and GS350 back-to-back, then:

ES longer more ungainly front overhang.
FWD-based often heavier power steering.
Often lower steering gearing.
Larger turning circle and lots of 3-point turns.
Slower turn-in.
Typically, less steering feel.
Steering self-centers due to large trailing angle for FWD stability.
​Also, self-centering under throttle known as torque steer.
Less terminal grip, as cheap single lower link front MacPherson strut cannot keep front tires at right angles to maximize grip.
Plus, more weight on front tires for less front-end grip.
Under throttle, more understeer.
New 7ES has suspension that is surprisingly firm, tied-down & tiring!
Though more rear legroom.
7ES newer design with kick opening trunk, scratch pad, touch screen, ACP and Android Auto etc.
Though plastics, leather, stitching, side & rear sun sunshades all budget quality.

RWD-based GS often:
Shorter front overhang.
Lighter power steering.
More direct gearing.
More steering feel.
Tighter turning circle & lots of U-turns.
Sharper turn-in.
When exiting corner, steering wheel does not tend to self-center due to less steering trailing angle for less stability requirements.
Under throttle, steering wheel will not self-center, known as torque steer.
Under throttle, rear oversteers to tighten turn.
Higher terminal grip, as front double wishbone keeps front tires at right angle to road to maximize grip.
Less weight on front tires for more front-end grip too.
Bigger trunk.
Higher quality plastics, leather & stitching.
Superior quality side & rear sunshades.
Older model, hence no scratch pad, no touchscreen, no ACP nor AA, no kick opening trunk, and no panoramic sunroof etc.