SSDS are of several types like SATA, PCIe, M.2, U.2, mSATA, SATA Express and many more. All of them are pretty fast if compared to normal HDDs which are commonly used even now because they are fairly cheap.
Links for some Cheapest and Best available ssds will given at the Bottom to checkout.
Which SSDs are the fastest -
M.2 SSD - this ssd connects directly on the motherboard without any cable hence making it the fastest and a little costly option. M.2 ssd is of 2 types -
M.2 Pcie and M.2 Sata - M.2 Pcie (M.2 Pcie Experess) leaves M.2 Sata SSD in Dust. Pcie SSD has lighning fast speed of 20G/s while Sata SSD only has a speed of 6.0Gb/s.
How much space do you need -
Ssds are bought for 2 main reasons - To use as Boot drive / For Games
If you are planning to use the SSD for games - Then Sata or M.2 Sata is the best option as it is more affordable and most games cant even use higher transfer speed than 6Gb/s..
Here are some SSDs with the Best Price Available on Amazon
M.2 240gb Sata ssd - WD 240gb M.2 Sata
M.2 480gb Sata SSD - WD 480gb Sata SSD
M.2 250gb Nvme ssd - Kingston 500gb M.2 Nvme SSD
M.2 500gb Nvme ssd - WD 500gb M.2 Nvme SSD
Normal Sata Drives - WD Green 240gb Sata