It all depends on what type of player you are. I'm a passive-tanker. Which means I always love to defend and research everything, and then build a huge fully teched-up army and crush the enemy. Unfortunately, GalCiv2 (and Galciv1, for that matter) do NOT allow my type of player to win militarily, at least not just by military means. The problem with military victory is that you simply cannot let your guard down - you always have to press on the attack. The AI is relentless and brutal, you have to keep attacking and building like crazy. This is WAY too frantic and aggressive for my taste, I don't like this sort of play style. I'm usually crushed before I even get to defend myself, because I always think: "My ships will be useless without this tech. Oh, and this tech might be nice. And this one, too, and that one, as well, and I just got to have this one and...oh, the Drengin are knocking on the door....". By the time I'm building my ships the enemy is so far more advanced than I am (they use brute force and downright intimidation to get their techs).
Therefore, the only way for us tankers (or turtlers) to really win the game is by wheelin' and dealin' and backstabbing everyone behind the scenes, it's quite easy and fun, and can keep you alive and well in the most dire situations. Pick a custom civ, give yourself A LOT of diplomacy bonuses, research nothing but the diplomacy tech-tree (the yellow one) and you'll be fine. Just offer a stupid little tech to another civ in exchange for alot of money, or convince them to kill each other while you build up your defenses, anything goes. Use the diplomacy screen nearly every turn. I usually let others do the fighting for me. At higher difficulty levels it's a bit more tricky, since the computer civilizations aren't so gullible, and you'll have to think deviously. This is quite fun, and it's a real treat and a very viable way to win through a diplomatic victory, or a technological one, even though a technological victory is much more difficult to pull.
In my last game I was up to my neck in defending my borders from pirates and the Yor, while trying my best to keep the HUGE yellow stain on the mini-map to engulf me from the other side (the Arceans had a REALLY huge influence power - I had to build influence bases like crazy). It was a race against the clock until, finally, I saw that "1 more turn to technological victory" in the left corner of the screen and I've won. Barely. It was really fun
Don't be fooled - the influence victory path is very much like the military victory one. That's right, you're not using any weapons, but the play style is basically the same - rush, expand like crazy, and keep on spreading your "disease" all over the galaxy. Too aggressive for me, as well.
Economics, are...well...troublesome. Why? Because I can get so much more money with a few well made diplomatic trades and a few strings pulled than I can with slowly building my economy and trade-routes. And even though economics are better on the long run when it comes to currency power and buying empires off, it's quite hard to start your little Universal-Mitrosoft there when at the beginning of the game empires keep knocking on your door. Diplomacy can deal with most problems early on until mid-game. Diplomacy major disadvantage, however, is on the late-game, when everyone has alot of tech and its becoming increasingly difficult to pull strings.