That latter variation mentioned by TwoMove is the current hot trend in the 4.Qc2 variation. Black seems doing fine, but...
Oh, and neitter the Slav, nor the Nimzo are "easy". OK, they have less theory than the KID, but both are very demanding, positionally-wise (especially the Nimzoindian).
Openings with Black against 1.d4 which are both reliable and light on theory, are just two: QGD/Lasker and QGD/Chigorin. QGD accepted is cool too, but it has little transpositional value.
Don't believe in objectively best opening either. Objectively a position is either drawn or won. Software like Houdini finds resources in apparantly difficult positions, thats how it gets its name. So software is showing practically any opening is playable. So what remains is what the human player feels comfortable playing. Back in 2003 gave up playing Nimzo/Queens Indian combination because hadn't been playing OTB much, it was a lot of theory to cover especially with lines were white didn't play c4 to deal with too. Lots of different pawn structures to master, this is sort of a strength of the Nimzo too, but when limited in time... The main reason though was the classical Qc2 was very popular. A typical solid d4 club player choice avoiding pawn structure weaknesses like the plague. At the time chould not find a response really liked, that gave lively play but felt sound. Again if I chould calculate like Houdini might have found one of the more unbalancing lines like Qc2 Nc6 playable, but I don't and therefore didn;t.
Recently in top level play 1.d4 Nf6 2c4 e6 3Nc3 Bb4 4Qc2 0.0 5a3 BxN 6QxN d5 has appeared were black has been able to use development lead in lines like 7Bg5 c5 8d4xc5 d4. So feeling it might be worth the work of taking up Nimzo again.