Thanks to JoeZ, pacoverflow, isaacg, Sp3000, Novarg, Ram, h34, and all those who contributed answers that have since been beaten. I've accepted JoeZ's answer since he has the most mentions on the list below, but this puzzle isn't 'closed': if you can beat any of the words on the list below, post an answer and your name will be mentioned!
For each letter of the alphabet, A to Z, find a word containing the highest possible proportion of that letter If people come up with more than one word with the same proportion, the longest one wins.
- taking the letter A, the best I can think of is "taramasalata" (6/12), which trumps "banana" (3/6) because it's longer; "banana" in turn trumps "razzmatazz" (3/10), although the latter word might be a good candidate for the letter Z
- taking the letter B, "blubber" (3/7) is an easy starting point which can probably be outdone.
- only words of at least 5 letters
- no proper nouns (i.e. place or person names)
- only English dictionary words
- exclude words like "aaaaaah" and "hmmmmmm" which could in theory be extended as far as you like to get arbitrarily many A's or M's
Hopefully this covers all the loopholes, so that nobody will come up with 'silly' examples that need more rules to avoid!
This is an open-ended puzzle, which means the current best answer may be outdone at any time. Partial answers (not covering all 26 letters) are fine; you can make them cover all 26 just by copying from the "current best" which I'll keep updated below.
A: araara (4/6) [credit: TakingNotes]
B: bobby (3/5) [credit: pacoverflow]
C: coccic (4/6) [credit: isaacg]
D: dodded (4/6) [credit: isaacg]
E: peewee (4/6) [credit: Joe Z]
F: feoff / fluff (3/5) [credit: Sp3000 / isaacg]
G: glogg (3/5) [credit: Joe Z]
H: hashish (3/7) [credit: isaacg]
I: iiwis (3/5) [credit: h34]
J: hajji (2/5) [credit: Sp3000]
K: kakkak (4/6) [credit: isaacg]
L: lulls / lolls / lolly (3/5) [credit: Joe Z / isaacg / Novarg]
M: mommy / mamma / mummy (3/5) [credit: pacoverflow / isaacg / Novarg]
N: nanny (3/5) [credit: pacoverflow]
O: voodoo / booboo (4/6) [credit: Joe Z / Sp3000]
P: peppy / puppy / pappy / poppy (3/5) [credit: pacoverflow / Novarg / Sp3000 / Joe Z]$^1$
Q: duqqa (2/5) [credit: TakingNotes]
R: rarer / error (3/5) [credit: pacoverflow / isaacg]
S: assess / sasses (4/6) [credit: Joe Z / pacoverflow]
T: tatty (3/5) [credit: Sp3000]
U: muumuu (4/6) [credit: Joe Z]
V: vivid / divvy / valve / civvy (2/5) [credit: pacoverflow / isaacg / Novarg / Sp3000]
W: powwow (3/6) [credit: Joe Z]
X: xerox (2/5) [credit: Joe Z]
Y: syzygy (3/6) [credit: Joe Z]
Z: pzazz (3/5)
At the moment I'm listing ties, but please don't go too much over the top with new words that match old ones - if there get to be too many equal winners for each letter, I'll have to remove all but the first one.
$^1$ Nobody mention "pippy"! :-)