What "allocation unit size" should I use for a drive with a single NTFS partition?

If you are a "Standard User" by Microsoft's definition, you should keep the default 4096 bytes. Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. If you have lots of small files, then it's a good idea to keep the allocation size small so your hard drive space won't be wasted. If you have lots of large files, keeping it higher will increase the system performance by having fewer blocks to seek.

But again, nowadays hard drive capacity is getting higher and higher making small differences by choosing the right allocation size. I suggest you just keep the default.

Also keep in mind that the majority of files are relatively small, larger files are large in size but small in units.

In terms of space efficiency, smaller allocation unit sizes perform better. The average space wasted per file will be half the chosen AUS. So 4K wastes 2K per file and 64K wastes 32K. However, as Jonathon points out, modern drives are massive and a little wasted space is not worth fussing over and this shouldn't be a determining factor (unless you are on a small SSD).

Compare 4K vs 64K average case waste (32K-2K = 30K), for 10,000 files that only comes out to 300,000KB or around 300MB.

Instead think about how the OS uses space. Let's say you have a 3K file which needs to grow 2K. With a 4K AUS the data needs to be split over two blocks - and they may not be together so you get fragmentation. With a 64K AUS there are a lot fewer blocks to keep track of and less fragmentation. 16x the block size means 1/16th the number of blocks to keep track of.

For a media disk where your photos, music and videos are stored (files typically 1MB+) I use the biggest AUS. For a windows boot partition I use the Windows default (which is 4K for any NTFS drive smaller than 16TB).

To find out what the cluster size is on an existing disk:

fsutil fsinfo ntfsinfo X: