Tech rush!!!!!!!
I mean, tech can buff everything even itself so... But then again the other replies make good points like pops and tradable resources.
I feel like it does depend on your empire as well.

As few as possible. Fewer systems are easier to defend but gotta have a few planets.

Diplomacy is tricky but even small attempts can delay an early war. Unless it's a fanatic purifier, determined exterminator, or devouring swarm, then just pray you don't look too appetizing or weak...

All that means is there's plenty of room for growth! As well as finding new things!

The victory from either the red button, killing everyone or most points at endgame? A few... Sadly I keep not finishing games before getting to that point.

To a few people, playing Stellaris in a roleplay way with you having personal objectives that don't actually make you win is fun. Yet many are wanting to have the biggest numbers, even though that's still fun! It's hard to get people together who want to do a bit of silly roleplay.

The machine age one that made 3 diff machine transition trees for ascension. I think it added the arc furnace and Dyson swarms which were epic as well!

Remnants. Rogue servitors acting as if your human creators hecked up their planet and you decide to take care of the poor guys.

Chuu chuu chuu! =x