What mods can you not play KSP without?

[Cirocco]

the ones that I need? Just one: KER. Can't decently estimate delta-V and properly build efficient rockets without it.

Ones that I like (read: use)

EVE for clouds and just generally nicer looking planets

procedural fairings for coolness puproses

...

that's about it really...

[WWEdeadman]

There really isn't any mod I really need. I started playing 0.25 unmodded on my streams recently, and although I miss some mods, I can still play just fine.

However, the mods I miss most when playing unmodded are

-Kerbal Engineer (for dV stats)

-Kerbal Alarm Clock (for stopping time acceleration for me, and being able to do multiple things at once)

-All of the visual enhancments (EVE + Better athmospheres, Hot rockets, Cool Rockets)

[Walker]

I've installed quite a few mods but I couldn't play KSP without DeadlyReentry and ProceduralFairings, the game would be just too incomplete.

[Geneborg]

Must have?

- SelectRoot -> yes, its simple but crucial for me

- KAC (Kerbal Alarm Clock) -> not missing a node anymore, great, not speaking of all the other functionality

- Precise Node -> yea, finetuning a node, what a blessing

- RCS Build Aid -> so important if you do any docking at all, but more so for large objects

- DPAI (Docking Port Alignment Indicator) -> docking UI, maybe not the visually nicest mod, but so helpful

- Procedural Fairings -> Rockets look like Rockets finally, and even more important when playing with FAR/NEAR

- Deadly Reentry -> well, the name says it all

Then there are alot of other mods that are "nice to have", but without the ones mentioned above i don't really want to play anymore.

[Oafman]

Apart from all the obvious ones, some notable mods haven't had much of a mention:

RCS Build Aid - very useful in the VAB

Near Future - great range of parts. With KSPi too, I'm basically at the point where I hardly ever use LFO anymore.

Ambient Light Enhancement - very useful, especially if you're playing when it's sunny outside (in RL)

Infernal Robotics - moving parts are cool

And though it's been mentioned quite a bit, I have only recently discovered the awesomeness of Tweakscale.

[NathanKell]

There are some mods I consider essential--Mechjeb, Ed Extensions, PAD, RCS Build Aid--but only one I simply cannot play without. FAR.