Brownee
The idea of villains uniting after their own previous solo failures against them is generally better than this new one being the majorest threat. As shakey as their alliance seemed, it's like turning friendship back on those that used it against them in the past. That's as good an idea for the big climax as you can get for this series.
It's the execution that was just a bit dumb. Had there been hints through the season that it was Discord, it would probably have been fine, but as it stands it just kinda came off as an unnecessary twist. Not to mention he literally pulled two of them out of the practically inescapable prison they were already in just to encourage them to be villains again, that's a bit too major a hole to just ignore in being kind of dumb writing. Particularly with him having been a reformed villain and such. Doesn't even work with him being 'chaotic' as a silly test for Twilight, there's just very little sense in that action.
It's the execution that was just a bit dumb. Had there been hints through the season that it was Discord, it would probably have been fine, but as it stands it just kinda came off as an unnecessary twist. Not to mention he literally pulled two of them out of the practically inescapable prison they were already in just to encourage them to be villains again, that's a bit too major a hole to just ignore in being kind of dumb writing. Particularly with him having been a reformed villain and such. Doesn't even work with him being 'chaotic' as a silly test for Twilight, there's just very little sense in that action.