bombers, lots and lots of bombers

You could also build ogrovs if you were TEC(advent adjudicators suck against SBs though).  I am guessing from the sound of building a SB to counter a SB that you are vasari though.  Bombers are your best bet unless your enemy has lots of fighters and still are preferable if you can thin out the fighter cover to manageable levels.  You could also build a SB at a different location if you are vasari against tec or advent and let it upgrade until sufficient to kill it.  Large amounts of lrf or HC can also take down SB pretty readily especially when supported with support cruisers and a couple of choice caps.  Bombers are far and away the most efficient way though.

[_]-Greyfox

I presume this is a Vasari starbase we're talking about (since TEC and Advent starbases can be attacked from outside of their weapon range.  The moving starbase is definitely the most challenging to tackle, but there are a few options you have:

1) Brute Force:

The Vasari starbase lacks any area of effect attacks.  It has only so many weapons ports and can attack only so many targets.  Throw enough firepower at it, and you'll be able to bring it down.  Unfortunately, for a heavily upgraded installation the kind of firepower you need is positively enormous, maybe 1000 command of fleet (though if you have the right support units in tow you can pull it off with less).

The danger here is that if you underestimate the level of force needed, you could take catastrophic casualties.  In singleplayer, you can always reload (so this can actually be a good learning experience about the fighting power of starbases), but in multiplayer you get one shot so you have to do it right the first time.  Good units to take down starbases include the Ogrov and Cielo for TEC, the Guardian and Progenitor (must have shield restore) work wonders for Advent, and for Vasari the Jarrasul with nano disassemblers is a miracle worker.

2) Bombers

As Ryat mentions, bombers work very well against starbases.  Starbases cannot attack strike craft, and move slower than carriers.  Simply move your carriers away from the starbase and their bombers can pepper it without ever taking a single shot in return.  This is more difficult if the starbase has fighter or fleet support, but is still doable particularly if you have a numerical advantage.

3) Bypass

Sometimes, you don't need to take down the starbase at all.  Just run past it and attack other planets behind the front lines.  The AI rarely defends them well and you can easily gut its empire like this.  Sure, it will keep a few front-line planets under control thanks to those starbases, but it's no longer a credible threat and you can pick them off at your discretion.

You can also use culture to overwhelm the planet in question (assuming it is stationed at a colonisable one).

Assassin X

You'd still have to get past the planet to attack the rest of the empire, so unless it's the last planet you need to take out, culture really doesn't solve the problem.

Thanks for highlighting that, Darvin3. Culture is not an end in itself. Just an alternative to 'ignoring' starbases per se and still taking out planets. It can be particularly useful when dealing with heavily upgraded starbases that protect loss of planet ownership by bombardment, for instance. As you have implied, a lot depends on where the starbase is stationed relative to the enemy's empire. If you have enough firepower to take out the starbase, or if you can bypass it, then by all means those are the better alternatives. Often I find I use a combination of all of these strategies online to success. I just thought I'd mention culture since it was not brought up earlier in the thread.

Assassin X