It's been over four years since we last asked this question, so let's have a 2020 update. The memory landscape itself hasn't changed a great deal in subsequent years - DDR4 is still the standard with cheap-and-cheerful kits readily available - and you might argue that our needs haven't altered all that much, either.
I'm running 32GB of DDR4-3200 in my own desktop, and in truth that's possibly double what I actually need. How about you? Let us know how much memory is inside your PC using the comments facility below.