What is the most damaging thing a cylon can do in BattleStar Galactica on a turn he is revealing himself?
Most of the absolute best reveals I have seen typically involve either getting an executive order, or playing a critical situation, whilst abusing things like arrest orders.  Most recently, we had Laura Roslin, who had moved to the Admiral's Quarters with the intention of visiting the Ally there, receive an Executive Order from another player.  Roslin immediately played an arrest order with her first action, brigging that player, and then with her second action, activated the location, targetting another human, and then playing the new 6-strength yellow skill card that auto-passes locational checks in order to force him to the brig as well with no check.  That ended the turn of the unfortunate human; next up was... Laura Roslin, who played another arrest order, brigging the last human player, and then revealed as a Cylon (the shock!) damaging Galactica.  That particular game ended in a hurry as the two revealed cylons were now opposed by three inmates, and the damage mounted up fast and furious.  

The lesson:

NEVER XO the player whose turn is next, unless all cylons are accounted for.

The contrapositive lesson for the cylon:

Get an XO when your turn is soon or next for maximum carnage.

Other tips:


Use communications to move civilians out of their defended areas straight into raider fleets.
Make sure you use up any Once-Per-Game effects you have;  you lose them when you are revealed to be a Cylon.  That means re-roll successful FTL checks(Gaeta), force humans to decide badly on skill cards (Helo), force humans to lose on Legendary Discovery (Boomer), shoot the President/Admiral (Cally), etc.  
Brigging the admiral is more effective than brigging the president;  it throws the title in the air, and you may wind up with it.  If you do, squander the nukes :)
If you are playing with the New Caprica expansion, the most lethal thing you can do is jump Galactica away before any civilians are launched.  Watch for timely opportunities to either be the Admiral (very difficult, lots of groups will execute the Admiral on principle to try to find the Cylon) or be someone who can become Admiral by power (Ellen Tigh) or by succession (be next in line, execute the Admiral).  As soon as Galactica returns, jump out, which causes EVERY CIVILIAN SHIP IN PLAY TO BE DESTROYED. I've never seen a group survive that.
If you are playing with the Ionian Nebula, visit lots of allies.  You can set up negative consequences on powerful allies, and then recommend to the other humans that they be visited :P.
If you are playing with the Ionian Nebula, you can try to eliminate a key human player during the trial by dumping trauma tokens on them with certain trial-crisis cards.  Also, in the leadup to that point, the more turns you can force humans to spend in sickbay/brig, the more likely they are to lose their trial or be flat-out executed.
If you are Lee Adama, you can use your once per game to remove viper cover from civilian ships at a key moment.  Don't forget that every viper launch is a mini-XO for you;  if the player before you launches a Viper, you can use that to launch yourself, trigger your once-per game, wreck the humans viper coverage, and then on YOUR turn, carry out the rest of your nefarious plot.  


Overall, the key to a successful reveal is to either cripple mankind directly with big stat hits, or to gain an advantage by wrecking their hands/freedom of movement through execution or brigging.  Bear in mind that in the Ionian Nebula, the longer any given person is in the brig (or sickbay) the more likely they are to get executed or eliminated in the trial.