First of all I would like to point you in the direction of the TI3 wiki. There are plenty of veterans over there who will go into great detail and length on their favorite SC or mix thereof and why they chose them. You might even get some people talking about the SCs they have created for the game.
Here are my general rules for SCs.
Imperial I: This card is just about the worst thing ever, It completely imbalances the game by allowing a player to simply score 2 VPs for choosing this SC.
Trade I: Trade II has improved on this card in just about every way possible.
Technology II: Unless you want a low technology game, this card is better than the Tech I card in my opinion. It makes technologies reasonably able to be purchased and with the racial technologies from the SE Expansion, this card is the only way to go.
If in doubt, just use the SE Expansion SCs. They are a lot more balanced than the originals and tend to keep turtling to a minimum.
My favorite SC combinations:
Set 1: SE SCs with Bureaucracy.
Set 2: Initiative, Diplomacy II, Political, Logistics, Trade II, Warfare I or II, Technology II, Bureaucracy (this tends to have smaller fleets due to no secondaries allowing the building of ships.
Set 3: Leadership, Assembly, Production, Trade II, Warfare II, Warfare I, Technology II, Bureaucracy. This favors large scale epic battles and aggressive strategies with both Warfare cards and Production in play along with no Diplomacy card.
Set 4 (8 players): Leadership, Diplomacy II, Assembly, Production, Trade II, Prospector*, Warfare II, Warfare I, Technology II, Bureaucracy. You could also exchange the Prospector* SC for the Logistics SC. (Note that you can also throw in a second Tech card, but this gives the Jol Nar quite a boost.)
*Player made card. (We usually just use the Imperial I SC to show this card since we never use it in normal games anyway.) Primary ability: Receive 1 AC, 1 CC and 1 TG. Secondary ability: None.