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 i7-7700K $280Nvidia GTX 1070 $410Samsung 850 Evo 250GB $85
AMD Ryzen 7 1700 $294AMD RX 480 $265Samsung 850 Pro 256GB $110
Intel Core i5-7600K $210Nvidia GTX 1060-6GB $260Samsung 850 Evo 500GB $140
HDDRAMUSB
Seagate Barracuda 1TB (2016) $45Corsair Vengeance LPX DDR4 3000 C15 2x8GB $205SanDisk Extreme 64GB $38
Seagate Barracuda 3TB (2016) $80G.SKILL Trident Z DDR4 3200 C14 4x16GB $1,050SanDisk Extreme 32GB $22
Seagate Barracuda 2TB (2016) $60Corsair Vengeance LPX DDR4 3200 C16 2x8GB $215SanDisk Ultra Fit 32GB $10
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