Out of those the PS4 Slim.
The Pro doesn`t do anything the Slim doesn`t and costs more. I prefer PS games to Xbox (and Nintendo for that matter), so the S is out. And the Switch isn`t confirmed to have achievements yet so that`s out by default.
Technically, the Pro (and this is if we wanna be. really simple about things) is a better PS4. So for $100 more, it does everything a PS4 does but better.
To OP, I'll get the Pro, primarily cause I've been entrenched in the Sony ecosystem for longer than I can remember but also cause I feel it's the most complete all round home console package to get.
I don't understand the switch. The way I see it is that it's exactly what most here thought it would be. in truth even less, it's a handheld console that allows you plug it to a TV. only reason I can see anyone getting it is if they want Nintendo games.
As for the XB1, well..... I just don't trust MS. Never have and they seem to keep doing things to make me feel I am right not to trust them. Call me crazy, but if MS was and has been the dominant player in gaming for say the past 3 generations. I'm sure they would have found a way to royally fuck over the consumer by now.