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 i7-6700K $330Nvidia GTX 1070 $376Samsung 850 Evo 250GB $98
Intel Core i5-6600K $220AMD RX 480 $200Samsung 850 Pro 256GB $120
Intel Core i7-4790K $339Nvidia GTX 970 $275Samsung 850 Evo 500GB $150
HDDRAMUSB
Seagate Barracuda 7200.14 1TB $45Corsair Vengeance LPX DDR4 3000 C15 2x8GB $103SanDisk Extreme 64GB $28
WD Black 6TB (2015) $270G.SKILL Trident Z DDR4 3200 C14 4x16GB $545SanDisk Extreme 32GB $23
WD Blue 1TB (2012) $50G.SKILL Trident Z DDR4 3200 C16 2x8GB $121SanDisk Extreme 16GB $16
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