I would not necessarily say this is the best device for all OpenHAB scenarios, but it is the best device for my requirements/scenario.
I used to run a Intel J1900@1.99GHz Fanless ‘Topton’ ‘Industrial’ PC with 8GB of RAM and a SSD (Was purchased off Aliexpress) and that has not missed a beat in 4 years.
I was however looking for a bit more ‘grunt’ and upgraded to a Ryzen 7 5800U based machine a few weeks ago, with more memory, and also a PCI3.0 NVMe drive.
Both these machines I have run as headless and whilst I have not explicitly measured the new systems consumption, it’s ‘Down’ TDP is 10W.
FYI - the new one that I purchased was https://www.aliexpress.com/item/1005005237006516.html , but don’t view that as an endorsement from me, yet!! Whilst it works well, and seems well made, it’s only been a few weeks, so too soon for me to call it 
Anyway the original system I had was perfect for running OpenHAB, performed OK, but this is where my requirements start to digress from OpenHAB alone. I use this as a server to run ALL home automation related items.
Both old and new systems run Ubuntu server.
The following you will probably find on any site running OpenHAB and sitting on a PI quite happily:
- OpenHAB
- InfluxDB
- Mosquitto
- NGINX
There are a few utility gateways running on there too, but all pretty light-weight
- CGATE (A Clipsal Integration for Home Automation)
- Python based daemons for Zelio PLC and Selectronic Inverter Integrations
I use the following to run the Documentation WIKI for the house, and this can chew resources :
- XWiki
- Tomcat
- PostgreSQL
And just about to install the following onto the same server (one of my motivations for the upgrade - Felt like this may be the ‘step too far’ for the old system)
- EmonCMS
- Apache
- MySQL
The applications run super-fast on the new server, with local page-loads on XWiki & OpenHAB feeling almost instantaneous
So really comes down to your intent. If just an OpenHAB ecosystem, the PI’s will hit the mark. If you are including a wider-sphere of apps/utilities, then maybe consider something with a bit more power, and maybe a Sata based SSD or NVMe drive for reliability.
If you need GPIO/I2C etc, then this is not the system for you… (However I do use PI3B out in the field running MQTTANY on Ubuntu for that purpose).
You’ll find plenty of fanless ‘Industrial’ PC’s available on Aliexpress or similar, with CPU’s which range from slightly more processing power than a PI, to a LOT more processing power than a PI. Your needs and your budget will set where you land in that range Good luck with your search.