Just as the topic says, I'm interested to know what the people on the forum here feel are the most useful, if not the best, treachery cards. I myself haven't had a tremendous amount of experience playing OL, so I'd love to get some ideas. For the moment here's the ones I find myself using most often:
Red:
Lone Assassin - I find the pierce 6 on the master deep elf an excellent way to get in some damage on the high armor heroes, plus the possibility of maybe destroying an item!
Dark Brothers - Master dark priests, pretty much powerhouses that give you extra threat, and will probably dish out some curse tokens on the heroes.
Lone Troll - Hard to get these guys into decent range, but if you manage it, this guy can dish out some serious punishment with Bash and Sweep, especially if you can play an aim card on it.
Green:
Crushing Blow - No explanation needed here, as it's pretty much a must have.
Ambush - If you wanna get in a few more attacks and maybe a kill before the heroes slaughter your monsters again.
Danger - I find myself often adding in one of these cards if I have an extra point of treachery left.
Purple:
These are the ones I have the most trouble picking from, usually I will go for the "interesting" traps like Drugged Darts, Scything Blades, sometimes Dark Relic tho I find that a bit costly depending on when you play it.