Steve nicely summarized things above, and I totally agree. “Best” will depend a bit on what exactly you are doing and already have. I use an ES-9 and love it for what I do.
Note that a combination of either an ES-8 or ES-9 with some of the expanders is also an option.
Last, and only related to the topic, bear in mind that there are some additional considerations that play into building hybrid systems- specifically as it relates to clocking and latency. Plan out how you are going to structure your patches and systems and just be aware of those being issues you’ll need to consider going in.
One of the benefits of the ES-9 is that it can work very happily standalone with a midi controller or an iPad. Whilst I originally bought it to interface hardware with Rack (and it was the first module I bought) I now rarely do that and it works perfectly well as my main Euro mixer (I have it configured with two mono sends/stereo returns, all controlled with my old BCF-2K).
Yeah, dream scenario. Two questions: Can the iPad act as USB host? Because the ES-9 needs that right? And, is it just a matter of reading the MIDI CC values to use for the onboard ES-9 mixer in the manual, and then getting a controller app on the iPad and get it to send those CC’s?
Since I already have lots of ADAT capable gear I went with ES-3 and expanders. People pestered Os for years for USB converter modules so many ditch ADAT-only when they upgrade. I finally got my ES-3 a couple years ago lightly used for less than $150.
The ES-8 and ES-9 are both amazing and would be better for my purposes, but I’m poor and biding my time. Main issues have been that I’m gradually switching from an all Mac-based setup to using more Linux and iPad. My old Macs and Hacks are equipped with Firewire ports either natively or PCIe card, so I use my old MOTUs as main preamp, converter, and ADAT interfaces.
But my newest systems are a mini Linux box and iPad and neither have either Firewire nor ADAT, nor even easy ways to add them. So from a perspective of spending $300-500 for a nice new USB3 interface with ADAT ports, it could be cheaper and/or easier to just go with a new ES-8/9. Also I have lucked out that I was able to use my MOTU Traveler with Linux FFADO drivers, but MOTU compatibility is flaky and unsupported. In comparison, the ES-8/9 are both class-compliant and should run on pretty much anything with a fat and clean USB pipe.
Also, be aware that TOSLINK optical ports can carry ADAT signals, S/PDIF, or both. I like the ES-9 best, but I don’t have much gear with USB-C yet, and it’s TOSLINK only does S/PDIF meaning that it’s not directly an expander to my existing ADAT-heavy setup. So I prefer getting an ES-8 with its USB-A and ADAT ports.