Sony. They were really shaky at the beginning of last gen, but in the end, I use Sony consoles sooo much more than either of the others.
I love the 3DS, but even that, I don't think Nintendo's doing it well. It's taken them too long to get on the online bandwagon and friend codes are a hassle and Miiverse/Nintendo Network is so... weird? You can't add friends on 3DS, only on Wii U. I think it has to do with the OS but... still... whether there's a good reason or not, it still feels broken. Add in the region lock, which I finally caved on and bought a second 3DS (used, though, so I didn't give them more money <_<)... and yeah. I'm not happy with Nintendo, even if I enjoy a lot of games on 3DS. I HATED the remote controller for the Wii (it was so uncomfortable in my hand, it required a fairly specifically sized room to work properly as a pointer in games) and I also dislike the game pad controller for the Wii U, so I haven't really liked Nintendo for TV consoles at all since Gamecube.
Microsoft was my favourite for a long time last gen since they were the ones that got online right, but once I stopped playing games online quite as much, I didn't touch my 360 at all. I played it a little in the last month but I don't miss it at all and haven't bought a game for it in years. I have zero interest in getting an Xbox One so... they certainly don't get my vote.
But Sony? Like I said, they started off kind of shaky but I think they ended up so well by the end of last gen. I have more PS3 games than almost any other console and I still find myself playing it for hours everyday, there are still a lot of games I haven't played yet that I want to try. Honestly, once PS+ became a thing, I was a Sony girl all the way. I do wish they'd put more effort into the Vita, because I want that to be great, but whether they do or not, I adore my Vita and I think it's technically a much better console than the 3DS... it just needs the library. :(
I also kind of make my decision like "if I had to give up all my consoles and only keep consoles made by one of the three, which would it be" and the answer to that is a very firm Sony.