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Depends entirely what you want to use it for. Also, what model MBP15 you have - is it newish (i.e with TB3/USB-C ports) or older (with USB-A and some combination of TB2, TB1 and Firewire) ?
 
Samsung has the T5 (540 MB/sec) and X5 (2,800 MB/sec) (TB3 only so MBP 15 2016 or later) series of external SSDs. Price is based on how fast you want to transfer data.
 
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1tb external USB3 SSDs have come down in price.
2tb may be available, but of course will cost more.

You can either buy an "all-built and ready-to-go" drive like the Samsung t5 or Sandisk Extreme (there are others as well),
or
You could buy a "bare" 2.5" SSD and the enclosure of your choice, and make your own (which is what I do).

If money is no object, there are also external thunderbolt3 SSDs, such as the Samsung X5, but they are PRICEY for the larger sizes.
 
It's a beast. They literally don't break. I had the Rugged, and it fell down the side of a rock shelf, like 120 feet, and I thought there was no way the data was still on there. That was 4 years ago, and I'm still using it to this day.
 
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