Which letter of the English alphabet takes up most pixels?
Hmm, let's see:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
cccccccccccccccccccccccccccccccccccccccc
dddddddddddddddddddddddddddddddddddddddd
eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
ffffffffffffffffffffffffffffffffffffffff
gggggggggggggggggggggggggggggggggggggggg
hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjj
kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk
llllllllllllllllllllllllllllllllllllllll
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
oooooooooooooooooooooooooooooooooooooooo
pppppppppppppppppppppppppppppppppppppppp
qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq
rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
ssssssssssssssssssssssssssssssssssssssss
tttttttttttttttttttttttttttttttttttttttt
uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu
vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy
zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
JJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJJ
KKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKK
LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
QQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ
RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
W wins.
Of course, this is a silly empirical experiment. There is no single answer to which letter is widest. It depends on the font. So you'll have to do a similar empirical experiment to figure out the answer for your environment. But the fact is, most fonts follow the same conventions, and capital W will be the widest.
Gist with these character widths in a ratio form (W = 100) captured here using this particular example font:
https://gist.github.com/imaurer/d330e68e70180c985b380f25e195b90c
Further to Ned Batchelder's awesomely practical answer, because I came here wondering about digits:
0000000000000000000000000000000000000000
1111111111111111111111111111111111111111
2222222222222222222222222222222222222222
3333333333333333333333333333333333333333
4444444444444444444444444444444444444444
5555555555555555555555555555555555555555
6666666666666666666666666666666666666666
7777777777777777777777777777777777777777
8888888888888888888888888888888888888888
9999999999999999999999999999999999999999