Best external hard drive to back up MAC

I prefer the Mercury Elite Pro series from OWC (macsales.com); they haave fast SSD option, medium speed 7200 rpm spinning drives or slow 5400 rpm ones (don't bother with one of those). Enough choices for connections too and are recommended here in these forums as they are "Mac centric". And, it is always good to have one that has its own power supply rather than drawing all of it from the Mac - the Elite Pro mini can be used either way which is handy.

I personally like bus-powered drives that use a separate SATA drive. That may be useful if there's an issue with the enclosure. I personally use a Seagate Backup Plus bus powered drive.

Some drives are basically an all in one unit where the drive and the drive to USB controller are all one unit. If something is damaged (like a USB micro-B port then it might be difficult to get it working. If it's a combined SATA/enclosure unit or one that uses a SATA drive, it would be possible to extract the internal drive and then place it in a drive enclosure.

Perhaps more information on what Mac you have? There are a variety of different connection types. You posted on a MacBook Air forum, and they can have a variety of different connections including USB-A and USB-C. Depending on what you have, you might need an adapter or perhaps a specialized cable.

I'm not a big fan of the pre-built external drives sold in big box stores because I have no idea what I'm getting. Sure, it might have a brand name on the box but what's the USB chip set? What kind of drive is inside? Are the cables high quality? So I purchase a high quality external enclosure (typically from macsales or IcyDock) and a good quality bare drive (typically Western Digital Black for backups). I just recently retired 2 FireWire drives that are at least 12 years old because I no longer have computers with FW ports. I've replaced bare drives mostly with higher capacity drives, not because they failed, and since I put the new drives in an already existing enclosure, I saved money in the long run.