Woo I'm in the tiny minority.
First point, for me Dragonfall beat them all three, and I don't care it is quite smaller.
Second point Wasteland 2 DC is my vote, not Wasteland 2 vanilla.
Third point why:
Writing:
DoS has a weird almost crap writing, that's a huge burden, PoE writing has many laborious parts rather tedious, WL2 is solid on that, much better than both, Dragonfall even better.
Combats:
Sure DOS has a cool combat system alas combats design don't follow, too many combats are a groups of enemies thrown somewhere without much design. PoE well I prefer a lot Tyranny combats should I comment more? WL2 has plenty combats with a polished design, many don't use a flat area as two other including DOS, cover system provide a much better tool base to tune the design of each combat and make them different. Dragonfall is even better than all, all combats are well tuned, there's a lot of depth and possibilities.
Party building:
DOS looks complex on that matter until you reach level 10 and dig more the system, then you realize it's very limited because of many aspects, attributes design and skills schools design. PoE is from far the best of all for depth and diversity. WL2 vanilla was a half disaster, party creation was rather interesting but past that not much. But Wasteland 2 DC changes all, with some additions then the party building is very interesting, more than DOS, more or less on par with Dragonfall, but clearly doesn't match PoE on that aspect.
Exploration of a world:
PoE screwed up any single wild area, and failed create a world feeling. DOS didn't even tried create a world feeling only a collection of small areas. Dragonfall tried nothing on that aspect. WL2 is the only one that achieved create a world feeling, at least much better than all other.
Depth of exploration:
PoE did well only for some or most dungeons, and the big town, that's short. For sure DOS is unbeatable for first areas but past that the quality drops hugely, it's not bad from far but other areas don't compare well to most WL2 areas on that aspect. Dragonfall did well but it's missions so can't be compared to other on that aspect.
Quests:
There's good, great and weak for all. DOS has probably the better achievement through the puzzling, quests mechanisms, and choices on how achieve goals. PoE is uneven, WL2 in the same boat. Dragonfall is a bit special with the hub and missions design.
C&C:
DOS is plain awful on consequences, they tried so much allow kill any NpC that to avoid budget skyrocketing all the writing is done to avoid consequences of any actions. This avoid bugs and open to players many paths on how achieve goals. At end it's still a game very weak on consequences. WL2 and PoE don't do a fantastic job in that matter but much better than DOS. Dragonfall multiply small consequences and added a few major with great choices deeply linked to interesting topic, overall the best of the four on that matter.
And so on, but that's not the point, the point is more simple:
1. Dragonfall
2. Wasteland 2 DC
3. DOS
3. PoE
But don't be wrong I have big hopes for PoE2 I pledged.