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 i5-9600K $193Nvidia GTX 1660S (Super) $230Crucial MX500 250GB $45
Intel Core i5-9400F $145Nvidia GTX 1650S (Super) $170Samsung 850 Evo 120GB $78
Intel Core i7-9700K $300Nvidia RTX 2070S (Super) $500Samsung 860 Evo 250GB $43
HDDRAMUSB
Seagate Barracuda 1TB (2016) $43Corsair Vengeance LPX DDR4 3200 C16 2x8GB $66SanDisk Extreme 64GB $72
WD Blue 1TB (2012) $32Corsair Vengeance LPX DDR4 3000 C15 2x8GB $64SanDisk Extreme 32GB $30
Seagate Barracuda 3TB (2016) $85G.SKILL Trident Z DDR4 3200 C14 4x16GB $649SanDisk Extreme 16GB $28
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