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 $300Nvidia GTX 1070 $368Samsung 850 Evo 250GB $99
AMD Ryzen 7 1700 $280AMD RX 480 $390Samsung 850 Pro 256GB $132
Intel Core i5-7600K $212Nvidia GTX 1060-6GB $285Samsung 850 Evo 500GB $154
HDDRAMUSB
Seagate Barracuda 1TB (2016) $45G.SKILL Trident Z DDR4 3200 C14 4x16GB $718SanDisk Extreme 64GB $45
Seagate Barracuda 3TB (2016) $83Corsair Vengeance LPX DDR4 3000 C15 2x8GB $140SanDisk Extreme 32GB $22
Seagate Barracuda 7200.14 1TB $55HyperX Fury DDR4 2133 C14 2x8GB $140SanDisk Ultra Fit 32GB $13
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