Another fan here of the Samsung external T3 SSD (and its predecessor the T1)! Fast, small and easy to always have with you and versatile. I have just recently purchased a 12" MacBook with the USB-C port, and now I use the USB-C cable to plug the T3 directly into the new machine, while still also able to use it with my rMBP, utilizing the USB-C to USB-A cable that comes with the T3. Makes transferring files and folders from one machine to the other a breeze!
I also use desktop external drives as well for backups -- in the past I used G-Drives but over the past couple of years I've been going with Seagate and so far (knock-on-wood!) no problems at all. The 4 TB capacity is great for backing up and saving all kinds of stuff and for backing up from more than one computer as well. Redundancy is key here -- everything is on the two Seagate external drives plus the most important stuff is also on the smaller 1TB and 2 TB T1 and T3 SSDs, which I've been using as supplements to my rMBP and now the rMB. Prior to purchasing the T1 last year I also used a couple of 2.5 portable bus-driven external drives, too.