With Lords Warscythe's are king, had him detach from his Warrior unit and run into a squad of IG Walker things, minced them and managed to join back in with his unit as they stood there and shot stuff.
Scarabs for me are so much fun, you can have them running around the board with a 2+ cover save and when it gets to the last turn or so (If they aren't dead) can boost 'em over to an enemy objective, or can run into a unit and hold them there for a turn or 2.
Destroyers are nice for shooting up units and light tanks, have them shoot up a unit then have some Warriors finish 'em off while you focus on something else. They're also quite tough as well so, like with the scarabs if there isn't much to shoot up, turbo boost onto an objective, which is good because normally people put weaker and more shooty units on them on their own side.
Warriors, bread and butter really, can cripple tanks, take a punch and with a res orb just makes it funny. Having a unit Deep Strike with a monolith and shoot up some guys is really useful, have them come out near a larger unit and use the re-rolls to WBB from popping in and out of it.
With Monoliths it really depends on what you're fighting, against stuff like 'Nids have it start on the board and template the fools until they get close enough to have those D6 shots of win. Against stuff like an IG gun line, deep strike and have some guys pop out, perhaps Flayed Ones.
Tomb Spyders are a good support unit, I'm considering taking a 2nd one. Have them stay in the middle of your force so you can make use of that extra 12inches for WBB rolls, very useful, had a unit wiped out and moved to the other side of the board where they then shot the hell out of Marines. Don't forget, you can run them! Get it towards a tank that's just sitting about and rip it to pieces.
I'd say you should give Flayed Ones a go, they're really quite useful, even more so if used with Pariahs (haven't actually tried it, but heard it works well). Making units take LD tests on a 7 instead of it's normal LD (unless it's normally lower), then if they fail, hitting on 6's, very nice Wouldn't suggest H.Destro's, they're ok, but, haven't had much luck with 'em. Never really used The Nightbringer, and only used The Deceiver a couple times, normally just stick to Lord's.