The best is the MONTAGE 8.
If you are asking about an external controller, recognize short of getting a controller that features 8 Zones, has Aftertouch, a Ribbon, MW, PB Wheel, two Assignable Foot Controller jacks, a Sustain pedal jack, an Assignable Foot Switch jack, at least 8 assignable Knobs, at least 8 sliders, you will be compromising on something... not to mention an assignable method to control the Super Knob (FC is preferable).

If your controller sends on less than 8 Zones, you can still play the MONTAGE Tone Generator. By placing the MONTAGE’s MIDI I/O Mode = Hybrid, you can link as many as 8 KBD CTRL Parts to sound on a single selected channel, and you can play other Parts, directly.

So if your external controller only transmits on two Zones, you can play one Multi Part and one Single Part MONTAGE sounds, simultaneously.

If you are thinking of carrying a smaller MONTAGE (the 6 or 7) and getting a weighted controller, is the bulk of the two worth the weight difference. Versus a MODX8? Thinking the experience of ‘playing’ a MONTAGE via MIDI is the same as playing the MONTAGE would be missing the point. Highly recommended — experience the whole package before substituting an external controller!

... keeping in mind the MODX series drops a few controller sources. Ribbon and aftertouch.

A Kurzweil PC4 is 16-zone. It doesn't have a ribbon controller - but that's, for me, one of the first controllers I'd be willing to "drop" since I've lived without it before using the previous gen MO series (which also didn't have aftertouch). The PC4 has aftertouch - and this is something I am now less willing to give up as I have aftertouch-triggered events.

The PC4 also has some other interesting possibly complementary features such as a CC step controller. You may even get the PC4 to do some interesting knob automation (assignable knobs, ribbon, etc) using the CC step sequencer. I'm actually not sure if the CC step sequencer goes outside (I'm sure it does) or is just to target internal CCs. There's a bunch of sliders and knobs on the PC4. The sliders have a drawbar mode so they reverse. Which may be useful if you can assign them for external control while reversed. It's also a synth - so there's complementary sounds and a 6-op FM engine for more (and more) FM. 256 note polyphony on the tone generator - which can be useful in using as a complementary tone generator.

There are other options out there - but this is, in my opinion, the best "bang for buck" if a complementary synth is of use.

A "dumb" MIDI controller (no synth) at 88 keys with 8+ zones starts costing about as much as this board does.

I personally don't like the mod/pitchbend above the keybed on the PC4. But this configuration matches the MODX8 and is still adaptable.

A used Motif XF fluctuates in price - but sometimes you find a deal on one at around this same price (or less even). It's not very complementary since it's the same sounds. The Motif XF offers the sampling and more advanced MIDI configuration - but not a whole lot of complementary stuff. You'll be "doubling up" on the basic core sound options. I would probably opt for a Motif XF6 if I wanted the sampler and other things (cheaper) for the studio but choose something more complementary in the sound department for an 88-key controller. Or go dumb for a lower pricepoint which wouldn't add any sounds and possibly also have less bells and whistles in the MIDI controller (knobs and control program-ability) department.

Not much available with a weighted keybed and 8 midi zones. I would recommend Novation SL MkIII but doesn't come with a weighted keybed. Or Kurzweil PC4 is very good.

The Native Instruments Komplete Kontrol S88 has similarities to the Novation SL MkIII (LEDs for each zone, for example), is weighted, and sports 16 zones.

I personally like some of the more advanced MIDI integration / control features in the Novation (vs Native Instruments) but ultimately went with Native Instruments for the zone flexibility.

The keybed of Native Instruments Komplete Kontrol is also very nice (I have the synth action) vs. your average MIDI controller. The 88 key S88 has a Fatar keybed. I haven't played on it - but it should be up to snuff when compared against your average MIDI controller. The downside is the lack of sliders and buttons and knobs and the rest. There aren't sliders. There's 8 knobs and 8 buttons. Not a ton of controllers (knobs, buttons, sliders, etc) on the Komplete Kontrol - so that may not be what you're after.

That said - retail of the NI controller is approaching the PC4. Particularly a PC4 on the used market.

I guess it's worth mentioning:

Yamaha Montage 8 = 63 lbs
Yamaha MODX8 = 30.4 lbs
Kurzweil PC4 88 = 28.8 lbs
Native Instruments Komplete Kontrol S88 = 29.1 lbs

... keeping in mind the MODX series drops a few controller sources. Ribbon and aftertouch.

A Kurzweil PC4 is 16-zone. It doesn't have a ribbon controller ..............

Kurzweil PC4 is equipped with a Ribbon Controller input. You can buy a dedicated Ribbon Controller just like it was for PC3 etc.
Apart from the advantages of Synthesis, it is one of the best equipped and best controllers on the market for me (like every Kurzwail PCx).

https://kurzweil.com/pc2srib_super_ribbon_controller/

2 feet worth of ribbon controller. Can optionally carve it into 3 sections for 3-in-1 ribbon controllers. Neat.

Velcro attach.

I know. I have this one with my PC3K8:)
I use it to control my MODX6. It works very well

Also there is Camelot Pro as well. It is designed as a solution for exactly these types of issues. Even if your keyboard has the connectivity built in (which our stuff does) it's great to have Camelot available for fast connectivity solutions. If you understand how to connect MONTAGE or MODX, or any MIDI gear up to a computer then you'll be able to connect easily with Camelot. I have it on iPad and it is really cool. Set ups, all the connectivity to control hardware and virtual instruments easily, sheet music and set lists (!), but it is really easy and nice to have connectivity solutions available on a separate touch screen.

Certainly our stuff has great controller solutions, but my last point is important. Having all the connectivity on a separate screen is pretty cool, and if you need to quickly connect and reconnect sources it's a godsend. I know another person who uses Camelot on a Microsoft Surface Pro and loves it.

There is a reason why we are running a promotion on this software: It works, it's affordable and an ideal connectivity companion for a live musician. If you want to check it out go here and see what you think:

Camelot Pro

Cam