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 i5-9400F $148Nvidia GTX 1660S (Super) $234Crucial MX500 250GB $45
Intel Core i5-9600K $210Nvidia GTX 1660 $210Samsung 850 Evo 120GB $150
Intel Core i7-9700K $340Nvidia GTX 1660-Ti $260Samsung 860 Evo 250GB $49
HDDRAMUSB
Seagate Barracuda 1TB (2016) $44Corsair Vengeance LPX DDR4 3000 C15 2x8GB $65SanDisk Extreme 64GB $88
WD Blue 1TB (2012) $40G.SKILL Trident Z DDR4 3200 C14 4x16GB $612SanDisk Extreme 32GB $48
Seagate Barracuda 3TB (2016) $75Corsair Vengeance LPX DDR4 3200 C16 2x8GB $80SanDisk Ultra Fit 32GB $14
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