These are just basic strategies {MMD} used at the begining of each map. They're fairly basic, but they work.
Arachnid 2:
Aliens - Rush the top, human bases up there don't last forever, but they can be hard to crack with a good defence.
Humans - Get to and secure the top ASAP. Build a base up there immediately. Never move under the stairs.
ATCS:
Aliens - Rush the hall. Jump that turret and grab some kills at the nodes.
Humans - Defend the hall and build backup turrets around the reactor and spawns. Rush the outside and take down the lower defences in alien base.
Karith:
Aliens - Rush human top, if you move quickly you can prevent any building up there and secure the win. If you can sucessfully do this you shouldn't require a move. If you do move, get to the dark staircase, not the elevator room.
Humans - Secure the top. If aliens break through there a single goon or marauder can kamakaze your base doing a lot of damage.
Niveus:
Aliens - Pull off a move, anywhere, preferrably the box room or window room, but anywhere is better then the default location. Rush the human's side entrance and pin them up in their default room while feeding off stray riflemen and builders.
Humans - Prevent the aliens from moving. Get building on the side entrance, or move to the red room or four door room. Lamer spot is also an option as long as nothing larger then a dretch is roaming around the map.
Nexus 6:
Aliens - Beef up your base and rush, there's not much else to do here. Moves are not reccomended because any competent team can defend the starting position.
Humans - Beef up your base and rush, there's not much else to do here. Moves are not reccomended because any competent team can defend the starting position. Forward building in the halls to the left is possible, but not reccomended on servers with lower buildpoints unless you intend to make a complete move eventually.
Tremor:
Aliens - Rush back entrance ASAP. Taking the lone turret is easy enough, then their base is wide open to feeding. Donate to teamates for a goon and it's over. Never put the Overmind in the boxroom unless it's absolutely needed to stay in the game. Otherwise, it's suicide. An egg/booster combo maybe, but that's it.
Humans - Cover back entry and build turrets in front hall ASAP. Then pansaw-rush their spawns. If aliens take tunnels, grenade the heck outta them. If still stage one, credit camp at the tunnel entrances till they decide to come out.
Transit:
Aliens - Go go go! Keep one or two grangers at base to beef up the acids or move while the dretches swarm like bees to honey. The default human base is completely lacking of defences, an armoury, and a medistation. The trickjumping to get there on time is well worth the trouble if you get to the human base before any significant progress has been acieved.
Humans - There's four or five possible base locations on Transit that are accesable from the default fairly easily. Other then the default, a good base can be constructed on the platform by the water towers, on the water towers, on the top level of the pit located to the immediate left of the default, or the blood room. I, myself, suggest that you immediately build defences and save a move untill S2, since no matter where you put the reactor it won't power anything if there's nothing built.