The Hardware - openHAB Community category is probably better suited to ask such a question.
But for running openHAB, I‘d use a Raspberry Pi with openHABian unless you don‘t have any other server already at home or need the power of something more powerful than a Pi.
Moved to Hardware/Server
I’d guess it’s a fake account anyway.
the cheapest NUC
is a newbie not allowed here?
You’re welcome of course.
Please familiarize yourself with the forum rules and documentation before posting though, please.
We have a number of fake accounts that post similar questions to not be right away detected as such.
FWIW, I have just ordered one of these (I have no financial interest whatsoever): https://www.aliexpress.com/item/1005004302428997.html
It’s pretty much ideal from my point of view, but of course people have different requirements. In my case:
This device will replace my now aging firewall, but I will try running openHAB on it first. If that works out well, I’ll get another for openHAB (and one as a HTPC but that’s not really relevant here).
If you do not have existing server, I recommend Raspberry Pi 4 as well:
We hope that Eben Upton will be right in his last update video:
In summary:
Q1 2023 1M units, lowest ever quarter
Q2 2023 2M units, 2nd highest ever quarter
Q3 2023 3M+ units…
Q4 2023 unlimited…
Check daily https://rpilocator.com/ for stock updates to get one you needs.
For what it’s worth, I also wondered if your first post was from a bot. 
Since we’ve established that you’re a human (or, at least, sentient), I’d also recommend an RPi4 if you can get one. However, an RPi3B+ will work just fine if you have one available.
Some folks run openHAB on Windows or MacOS, and most (not all) eventually move to Linux. Since there are far more of us using Linux in some form, it’s easier to get help and find solutions.
Power consumption is the main reason to choose an RPi or another single-board computer, but an old, unused PC can also work. I recently installed DietPi on a 13-year-old Dell laptop and it runs surprisingly well. The openHAB 4 Milestone can be installed from the DietPi interface, but I haven’t tried it out yet. I’ve been experimenting with other things in DietPi.
An old PC will consume much more electricity than an RPi4, which adds up over the course of a year. As stated initially, I think an RPi4 is a better solution for openHAB, but there’s something to said for giving new life to old equipment.
You can run it on an old laptop and then you have a built in UPS and also a screen.
I actually took the battery out of my laptop, because it was old and I fear it swelling as the cells break down. The good thing is that most laptops can run without batteries, unlike phones and tablets that typically only draw power from the battery.
It has an SSD so I’m less concerned about corruption, but if I do keep it running as a server it’ll definitely go on my UPS.
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:
There are a few utility gateways running on there too, but all pretty light-weight
I use the following to run the Documentation WIKI for the house, and this can chew resources :
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)
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.
Since the Raspberry Pi 4 is not in stock all over the world, what is the best alternative if I need a new device ?
If you can wait a few months, then that might not be an issue.
Alternatively, you could look for a Raspberry Pi 400. I don’t know where you are, but in some regions they never went out of stock.
I also saw this article, but actually I want to get rid of the SD Card and I was looking to Odroid/Orange Pi, because they support eMMC. Or any orther device that is supported by Armbian
It’s a persistent myth that eMMC is any better than SD but wrong.
And with Armbian, you’ll be missing all the benefits of using openHABian.
Long story short, anyone can choose to run OH on whatever he likes and is familiar with, hence the official recommendation.
But if you have no strong preference, in most common usage scenarios, the best device is a Raspi 4.
Thanks for the link to the post of SD card, SSD, eMMC… I’ll wait for the next batch of Pi4’s and maybe already have alook for a brand new SD Card. I do have some old Pi3’s laying around, but I think I prefer some faster ones.
Looks ideal for me, too. Which version did you order? Do you see an advantage in buying the 16gb-version?
We been just arguing over that in other topic. Its not a myth as there are less hops between CPU and onboard eMMC than between CPU and SD card. You might get a SD card reader behind several electronic components. For that reason eMMC will be more stable than SD card, especially when you include various environment conditions. While eMMC and SD card might be based on the same flash memory kind, they don’t have to.
There is a reason why embedded systems heavily rely on eMMC and not SD cards. This reason is stability of such storage. Believe me, if SD cards would be equal to eMMC then embedded computers would use them on daily basis as boot drives, cause it is cheaper to solder a card reader than to get there a emmc. For some reason its not a common practice to get your smart speaker with SD card thrown into a box.
Let’s not have a repeat of it here. You’ve both made your points in the other topic, and I think it’s reasonable to call this an “agree to disagree” scenario. Time for everyone to move on.