Characters that don't start with the letter "Q".

kogmaw 2 sec ult cd or even less.

its a tie between kat, darius, and zil imo

Ryze, his passive seems strong with Karthus ult. Better than Rewind maybe.

If were straight up replacing the ult entirely, then id have to say either veigar, or an ap assassin who does not particularly need their current ult to do their job. Maybe leblanc. Idno.

This could be interpreted multiple ways. By everyone having requiem do you mean everyones ult turns into the spell requiem with its cooldown etc included? the spell becomes like requiem (eg. it now targets 5 instead of 1)? or some weird hybrid? OP needs to elaborate.