I read the whole region-discussion from a few pages ago and wanted to add my own opinion but never got around to it so... I'll do it now. Unova is my favorite region. I love everything about it. The variation when it comes to environments is wonderful and the best in the series. It has literally everything, from fields to seas to volcanoes to bridges and even seasons! I also love the layout. I am of the very unpopular opinion that linearity in the games is a very good thing that the games benefit from, compared to non-linearity that offers zero benefits but causes massive problems. This is regarding the main storyline, not the post-game. I also love the Pokemon distribution in both Unova installments. Then Generation 5 is my favorite so I am automatically biased towards the region it offers as well.
I like Pokemon Battle Revolution. While it was mainly a 3D battle simulator for Gen 4 and a substitute Battle Frontier for D/P, it was still a lot of fun to play. And rather challenging too, beating 100 battles in a row at Courtyard Colosseum was quite the trial, and beating the higher ranks at Stargazer Colosseum took me many tries. The graphics in the game were great, and the music was amazing. The game didn't do a lot but most of what it did, it did well.
I don't think G/S/C & HG/SS has the best post-game in the series. I don't even think it is good. It is bad. Very bad. While there is a large area to explore, it just isn't interesting at all. There is no story in Kanto whatsoever, just a very minor thing regarding the Power Plant, which was boring to play though and just another example of uncreative roadblocks in the games. Kanto was way too open and not linear enough so there couldn't be any level progression at all. And the wild Pokemon started out at too low levels to begin with, the trainers were also too weak. Beating the Gym Leaders weren't interesting either due to the lack of story and proper characterization. The areas and environments were uninteresting, the wild Pokemon distribution offered very little new and interesting as almost all of the wild Pokemon had already been encountered in Johto. Overall, it was a bland and boring mess compared to what it could have been, or what it was in R/B/Y & FR/LG. However, I wouldn't say it is the worst post-game in the series, but it isn't far from. Should Game Freak decide to make a proper post-game again in the future, I hope they use B2/W2 as a basis as I think these games have the best post-game in the series by a long shot. They are also a good and recent example that Game Freak can make a good post-game if they just want to.
Not sure if this is unpopular, but... I really like Gothitelle. I always preferred it over Reuniclus (which I also like, but not as much) and I definitely like it better than Jynx and Gardevoir, the former is interesting but not one of my top favorites, and the latter is my least favorite out of all existing Pokemon (maybe this is also unpopular?). Gothitelle is awesome all around though. I have zero interest in fashion but I think gothic lolita looks awesome so a Pokemon based around it gets my approval.
Lastly, another which I am unsure about whether it is unpopular or not. In addition to Generation 5 being my favorite, I really like both B/W and B2/W2. I believe this is (at least somewhat) unpopular because based on what I have seen on several Pokemon forums, people generally seem to like/prefer one 5th Gen pair but dislike or not care much for the other. I really like both of them though, B/W mainly for the story and characters, the new start feeling and for bringing me back into Pokemon once and for all, B2/W2 for having the best and most content in the series and being extremely fun to play in addition to follow up on everything B/W did and continue doing it very well. I also loved how much actual new content and features there were, much more so than what Yellow, Crystal, Emerald and Platinum had compared to their first pairs. Or for that matter, more than what FR/LG, HG/SS and OR/AS had to offer compared to their originals. There are many other reasons as for why I love both 5th Gen pairs, but these are my main ones.
Edit: Fixed spelling mistakes and made some small elaborations.