There is a total of 20 "Most Wanted" Ghosts, I will start the list with the easiest and move down.
The Book of Pain - Requires you to SUCCESSFULLY complete a job (any type) in a Library level.
Captain of the Guard - Must Kill The Book of Pain first, then rack up 20 Paper Constructs or Book Centurion kills.
Olga the Terrible - Must Kill Captain of the Gaurd first, then complete 5 Library matches.
Dredge - Must Kill Olga the Terrible first, then complete 10 Theives jobs in a Library level.
Times Square Ghosts
Smelly Ned - Requires you to complete 5 Containment jobs in a Times Square level.
Dolnansky the Foreman - Must Capture Smelly Ned first, then you need to collect 50 power-ups.
Grimgreave - Must Capture Dolnansky the Foreman first, then rack up 20 Gargoyle or Stone Angel kills.
Binky - Must kill Grimgreave first, then complete 10 Survival jobs in a Times Square level.
Grundel - Requires you to complete 5 Protection jobs in a Museum level.
Sharpshooter Bailey - Must capture Grundel first, then rack up 15 Civil War Ghost captures.
The Maestro - Must capture Sharpshooter Bailey first, then complete 10 Survival jobs in a Museum level.
Grand Pappy Sargassi - Must captue The Maestro first, then complete 15 jobs (any type) in a Museum level.
Mad Mad Minimo - Requires you to earn $50,000 in a single match in a Cemetery level.
Darkest - Must capture Mad Mad Minimo first, then captue 15 Black Slime Ghosts in a Cemetery level.
The Clawed Menace - Must capture the Darkest first, then rack up 50 Cemetery Crawler kills in a Cemetery level.
The Uprising - Must capture The Clawed Menace first, then complete 10 Survival jobs in a Cemetery level.
Slimers(Can be captured on any map)
Rotten Slimer - Capture 10 Slimers in any of the levels(excluding the Slime Dunk jobs)
Wee Slimer - Must capture Rotten Slimer first, rack up 3 wins in the Slime Dunk jobs on ALL LEVELS.
Polar Slimer - (Currently Glitched) Must capture Wee Slimer first, then rack up 20 Slimer Dunks on any level.
Glutton Slimer - (Currently Glitched) Must capture Polar Slimer first, then play a total of 60 jobs (any type and any level).