Notes:

I went ahead and started cataloguing memes that are about the letter itself or close to it. (Certainly we could find memes starting with all the letters, but that's too easy.) These can all be found on KYM but only with the correct number of letters for some of them.

A: aaaaaaaa (8 a's)
B: B emoji
E: Markiplier E
F: Press F to pay respects
G: r/ggggg (5 g's)
K: A meme from the 1800's: https://en.wiktionary.org/wiki/oll_korrect https://www.straightdope.com/columns/read/503/what-does-ok-stand-for/
T: T-pose
U: No U
Y: Y tho

EDIT: Others have alerted me to the existence of other letter memes, listed below. Not sure if I can edit this image though.
H: h
D: Give the D
L: Take a loss
W: Take a win
M: see Markiplier E
C: see Markiplier E