Probably the slav is the most straightforward. If white doesn't play critical lines with c4, nc3 and pressure on d5, black can just develop with bf5/bg4 etc without giving up the centre with d5xc4. Will give equality if not necessary exciting positions. No system approach works against everything, so will have to find something else against 1.f4 for example. If play the a6 slav can have the same aim of avoiding giving up centre with d5xc4 in the critical lines.
What's a solid setup against off beat systems
I guess it depends on what you play against 1.d4. For example if you play the KID against 1.d4 but use some ...d5 setup against this stuff you could get move ordered into something else. With most d-pawn systems White can usually play 1.d4 and 2.Nf3 without committing to anything.

Mostly the systems coming from d4. Colles, Londons, Stonewall. Reti even. But I also want something for some b3, f4, and such.
I know I can play a Slav, KID, or even Grunfeld setup against all these, which is what Im looking for, to cut down on learning, since most people dont play main line stuff at my level anyway. Most of this weird stuff stems out of d4 I find so Id like to pick one of those defenses that also work well to be a just general standard setup.
Just wondering what you guys play or what you think is more solid