Best format for external disk?

If the drive is going to be for data only, APFS is better for an SSD and HFS+ is better for a hard drive.

I found out the latter when I bought a new 3 TB external hard drive from OWC. Since every other drive was APFS, I figured it made sense for that one to be the same. But it mounted slowly, and read/write times were much slower than they should have been for a 7200 RPM drive. I called OWC and the tech person there suggested changing it to HFS+. The drive was clearly faster doing everything after changing it.

I guess this applies also to data disks:

"a rotational [APFS] device will never be able to provide acceptable performance as a production macOS startup disk"

https://bombich.com/blog/2019/09/12/analysis-apfs-enumeration-performance-on-rotational-hard-drives

That said, I have an old 2009 2.5" external APFS HDD where I occasionally test Catalina, soon Big Sur (internal SSD is Mojave). Yes, it is slow and takes about 4 minutes to boot.

Yup. I only used the phrase "for data only" because that's what the question was about. Otherwise, it depends on what you're going to do with the drive.

Let's assume you plan on creating two, or a few hard partition breaks on an HDD. If a partition is for data only, then yes, use HFS+. If you're going to install Catalina, then you have no choice but to use APFS for that partition since it's required for Catalina.

But I assume you already know this.

Good info. One of my external drives is a 1 TB SSD in a USB-C case. That one is broken into four hard partitions and all are APFS. I did try HFS+ for the one that just has backup data on it, but APFS was obviously faster.

Another has a 4TB hard drive, also in a USB-C case. Results were the opposite of an SSD. APFS partitions were always slower than HFS+.

You're welcome.