If we are talking Single Player then Diplomacy without a doubt.

First you need to research starbases on the Entrenchment tech tree.  For Vasari you need 2 military labs, for TEC and Advent you need 3 labs.  Then you need to build a starbase construction ship.  Vasari just uses its colonization frigate.  TEC and Advent have to build a special starbase construction ship, which is listed as a cruiser on the build menu.  To actually build the starbase, select the unit and then click the starbase building ability and then click on a location on the map where you want it to build.

Note that the Vasari starbase can move around in the gravity well and is often used on offense because it can knock down all enemy structures.  Also since it can move it can be built and leveled up away from an enemy starbase and then sent in to attack an enemy starbase.  One common tactic in online multiplayer is to try to build the Vasari starbase at an enemy's home planet, which is known as "starbasing".  This is especially effective if it can be done in the first 10 minutes of the game.

The safest way to kill an enemy starbase is with mass bomber spam.  Experienced players know how to (and prepare to) defend against that, however.  If you are TEC you can also build lots of Orgovs which can devastate starbases from a safe range.  Advent has its own anti-structure frigate (the "starfish") but it kind of sucks.

For online multiplayer, Diplomacy is definitely the place to be.  You can still find 4v4's and 5v5's there if you are patient.  There are also people on regular Sins, but it's mostly new players doing comp stomps and 2v2's with an occasional 3v3.  What's interesting is that few people seem to be playing Entrenchment.  (I think all of them purchased Diplomacy.)

Hey Dual_CoRed,  Welcome back.  Diplomacy includes everything that Entrenchment has, plus all of the Diplomacy features.  So go straight to Diplomacy!   Most people online play Diplomacy like they would play Entrenchment, and ignor most of the Diplomacy features.  But Diplomacy has one important difference: it doesn't allow feeding until the Eco guy researches a tier 3 Civic tech.  This greatly evens out early rushes and makes the game much fairer & competitive. 

Some of the Diplomacy pacts are quite powerfull, but they take a considerable amount of time & investment to get working. 

Also all of the artifacts in Diplomacy have been improved considerably, so they may finally be worth exploring for.  Other than that, Entrenchment & Diplomacy are identical, except for the additional Diplomacy features.

Oh, and the Distant Stars mod for Diplomacy seems quite popular, and people play it online on Sunday afternoon/evenings!