A Speed, price, capacity, USB version and weight.
The four main factors are: read speed, write speed, price and capacity. Speeds should be quoted in MBps and should be checked before making a purchase, some drives are painfully slow. The main thing to ensure is that the drive is USB 3.0 which is significantly faster than USB 2.0. Some manufacturers offer various bells and whistles like built in security or backup features but these are generally available as freeware downloads (e.g. Truecrypt and Syncback) so are not worth paying any extra for. Finally you might want to consider weight and physical dimensions, some drives are ten times heavier than others.

The Best.
CPUGPUSSD
Intel Core i7-8700K $370Nvidia GTX 1070 $370Samsung 850 Evo 250GB $100
AMD Ryzen 5 1600 $150Nvidia GTX 1060-6GB $269Samsung 850 Evo 500GB $115
Intel Core i5-8600K $260AMD RX 480 $280Samsung 850 Pro 256GB $161
HDDRAMUSB
Seagate Barracuda 1TB (2016) $40Corsair Vengeance LPX DDR4 3000 C15 2x8GB $145SanDisk Extreme 64GB $80
Seagate Barracuda 3TB (2016) $80G.SKILL Trident Z DDR4 3200 C14 4x16GB $869SanDisk Extreme 32GB $30
Seagate Barracuda 2TB (2016) $60Corsair Vengeance LPX DDR4 3200 C16 2x8GB $155SanDisk Ultra Fit 32GB $11
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