What are the best balance and survival mods? There's several...
Oh, and combinations are welcome, like say X mod is good for doing this but Y mod is good for that, so just use both (if there's not a conflict)

I'm looking for more challenging but realistic gameplay.

And more intense survival.

Vote 1 for Arwen's Realism Tweaks here. Been a satisfied customer since FO3. though i must admit to tweaking it a bit using XFO's accuracy changes and non-var damage modules as a base and heavily customizing it myself.

I recommend the http://www.newvegasnexus.com/downloads/file.php?id=38301. It is not as luxurious as the Portable tent I use, it's more for a survivalist but it's nice to be able to camp out and really get into the RPG aspect of camping. There is also a portable bed and campfire but I've never been able to get the bed to work, it flops around so I use the tent.

XFO + IWS for me

Arwens Realism Tweaks + Increased Wastelands Spawn (IWS).

This.