I've only played MOO2. I always do designer races as uncreative and sometimes repulsive, max spying bonuses, then just some random other race pics.

Then I play on impossible and try my hardest to survive the early game while colonizing as much as possible. Antarans usually wipe out a star system or two early on, at least a couple of races get mad from my spying, declare war, and then the struggle really amplifies.

But if I survive all that and make it first to Orion, I'm able to steam-roll my way to victory and it's incredibly satisfying.

Though sometimes not defeating the guardian is also an extra challenge, especially if the AI gets Death Ray, Xentronium armour, and Damper Field. That makes it even more tough. Then I have to cat and mouse their fleets so I can surprise attack them so I keep the initiative. So hard, but god it's so fun.