PC + WiiU or PC + PS4 are the best combinations IMHO (depends if you prefer the Nintendo or PlayStation exclusives), if you don't want more than 2 systems. PC + XBO or PS4 + XBO is a bit redundant.
But I'm happy with my current combination (GTX970-PC + PS4 + XBO + WiiU + Vita + 3DS + iPad Air + Android smartphone) ;)