Best macOS version for IIci?

[Crutch]

You can definitely run 8.0 on an 030. I’ve run 8.1 on my SE/30 with a Diimo 030 accelerator. You need a 32-bit clean ROM and a simple ResEdit hack. Instructions here courtesy of BMOW: https://www.bigmessowires.com/mac-se30-with-upgraded-rom/

 

[Byrd]

I thought I saw somewhere that there's a hack to get 8.0 onto a 68030.. It's a shame there isn't an active community making modern accelerator boards for these old Macs like they do for the Amiga.

You should be able to pickup a Radius '040 accelerator for the IIci, or even a PPC accelerator if you spend time looking around for a good price.  Accelerators for IIci machines are too niche as they only work in that machine, others will need an adapter so it wouldn't be a good candidate for a "modern" accelerator.