The only recent equivalent was the S90xs. Right now there still should be the 76 key version on the market. The S70XS.
I personly hope that there will be a succesor out soon. Probably it´s gonna be the S90DX.

What is the equivalent of the S90 in the current field of Yamaha Synths/Workstations? Weighted keys, great piano sounds etc?

The MONTAGE 8... Balanced Hammer w/Aftertouch, top-of-the-line sound engine, great piano sounds, full synthesizer.

The S-series always paralleled the Motif-series (Balanced Hammer w/Aftertouch, top-of-the-line sound engine, great piano sounds, no compromise synth engine). The Motif-series was built around the ISS (Integrated Sampling Sequencer), the S-series was a Motif without the ISS. The S-series, for example, had a playback sequencer only... and lacked the full ELEMENT editing found in the sister product line, Motif.
The concept was the S-series customer would likely use a computer to Edit (thus the tiny screen, but full Remote Control Surface implementation).

Currently nothing completely matches the S-series role. The MONTAGE/MODX share more of a Motif/MO-series relationship.
Both are fully editable, both have the ability to do it all right on board. The Performance Recorder allows you quickly document your musical ideas and drag n drop them into your favorite DAW for editing.

The other alternative is the CP88. THE CP88 is NW (natural wood) key, GH3 (Graded Hammer triple sensor action), combo piano. It features an Electric Piano section and a Sub section with a variety of sounds that go with acoustic and electric pianos for gigging. There is not the in-depth editing as found on synthesizers. They are different animals ( the MONTAGE 8 and the CP88 ), but those are the two products that sit at top-of-the-line level, like the S-series, and deliver contemporary sounds, killer pianos, and unprecedented sound quality.

From this point of view Bad Mister is surley right.
The Motif or now the Montage Line was/is always a bit more expensive than the S90 Series.
This is because the lack of some features mentioned above.
One big difference between the MODX and Motif/Montage/S90 is the action. You just get the GHS version in the MODX.
The CP88 is a totally different instrument like Bad Mister just said. A Stage Piano and not a Synth-based Key Instrument.

Thanks folks. I think the answer is that as someone who prefers a piano action is to go with a CP88, and at some point add a synth. Sometimes splitting the baby is splitting the baby.

The S90XS has a more similar keybed to the Motif series than to the CP series. The S90XS was sort of a scaled down Motif with even less bells and whistles than either the Motif or MO series. The MO series scaled back the keybed from Montage (among other things) - but the S90XS kept the keybed similar ("scaled up" ) to the Motif 88 key variation.

The CP series (88 key) was generally less in the tone generator department (less content) while the keybed was an upgrade vs. the Motif series. Upgrade by some -- more piano like might be a better way to put it.

When Montage came along - at launch - it was actually more like an "S" series piano than the Motif. In the sense that Montage dropped on-board sampling and sequencers just like the S series shaved away from the bigger brother of the Motif line. So Montage 8 got closer to being an "S" series than anything else in the current product line.

The CP is similar in limited editing, small screen, and limited tone generator content. But the keybed is different than the "S" series because the "S" series has been closer to Montage's/Motif's than anything else.

If you went for the CP - then you might still go for the CP even if they had an S95 (or whatever they'd call the Montage-era S series keyboard). Because today's "S95" would feel a lot like Montage's keybed vs. the CP88's.