Is Princess Luna everypony's favorite alicorn because they like her so much, or is she everypony's favorite alicorn because she's the only one they don't dislike?
Everypony hates Princess Celestia because they view her as a troll. They also say she doesn't do anything, which is not true. She defeated King Sombra, Discord, and Nightmare Moon a thousand years ago (even if she couldn't permanently defeat them, and the first two alongside Luna). She's the one who decorrupted Twilight enabling her to do the same to her friends and defeat Discord, she broke up all the chaos Twilight caused, and she fought against Chrysalis who only won because of how powerful she became from feeding off Shining Armor's love.
Although it is true she didn't do anything when Rarity was falling to her doom, but that could be because she wasn't fast enough at that point, thought the Wonderbolts could, and knew Rainbow Dash would. As to complaints that she didn't do anything about the whole thing with the cutie mark switching, she intended that to play out exactly as it did, knowing Twilight would be able to complete the spell. And King Sombra... again, she knew Twilight would find the Crystal Heart and make the decision she needed to make. As for Nightmare Moon, it's a common misconception that she didn't believe Twilight; she said she saw the signs of her return and knew only Twilight could defeat her, because only possessing half of the Elements' power without her sister, Celestia couldn't draw Luna out from Nightmare Moon on her own.
Also, Luna doesn't do anything in the fight with the changelings, yet this didn't generate any hate for her.
Everypony hates Princess Cadance because she has no flaws, and because she is the third alicorn in the series when people wanted Celestia and Luna to be the only alicorns. Well, look at it this way: The first three episodes she appeared in (four if you count the flashback) were epic Disney quality episodes (though some feel differently about the latter two), and came packed with the two most dark, malevolent, downright creepy villains in the show. She could be that "powerful ally that comes in later", like MarineAngemon. She also defeats Chrysalis and Sombra (alongside Shining Armor the first time and the crystal ponies the second), killing the latter and maybe the former, comics aside, but I guess that might cause some to view her as overpowered, and outshining the Mane 6, not to mention blaming her for Sombra losing any chance of returning for more screentime and lines, same with Chrysalis if they ever confirm that.
But if she wasn't an alicorn, Spike would be dead. When King Sombra was riding a dark crystal to spear the baby dragon, Shining Armor threw Cadance at him in an Incredibles moment, and she got him and the Crystal Heart to safety with her wings. Also, she and Twilight probably would have fallen to their doom otherwise during the end of the mining cart in A Canterlot Wedding. Good, then that alicorn I hate so much would be dead. That's dark, man, who would wish that on her, and besides, it wasn't just her in that scene but Twilight too. I'd have preferred if she had died to what happens 13 episodes from then.
Which brings me to that. I've already said it in great detail under my first blog, so I'll just cover the basic points. Some hate our little Twilight Princess because of her wings, some because of her title, some because they don't feel she did anything to deserve it, some because they feel that the "princess" thing is a symbol of regression into the "girly stereotype" of G3. We should be happy for Twilight, maybe princess in Equestria is like king in Middle Earth (like Aragorn). And Seasons 1 and 2 did all right with TWO princesses already, plus there are how many princesses in Zelda? And she did quite a bit for that transformation. She defeated Nightmare Moon by letting true friendship into her heart which she couldn't do before, she decorrupted her friends enabling them to defeat Discord, she proved instrumental in the defeat of Chrysalis, she made a critical decision that enabled Spike and Cadance to destroy Sombra and save the Crystal Empire, thus passing Celestia's test, and she finished a spell that Starswirl the Bearded couldn't. Why her? Because she is an exceptionally powerful unicorn, and all Celestia needed to do was make sure she was worthy of the title. "You have proven that you are ready, Twilight." As to why a fourth princess would be necessary, there are many mysteries of Equestria the show never elaborates on.
Now people are entitled to their own opinions, but I feel what is should be generating is: a.) Twilight Princess jokes. As in, the Zelda game. When I played through the game for the first time after watching MLP, I thought Twilight Sparkle every time they said "Twilight", and more than once "If only Twilight was a princess, then we could call her by that name". And I can totally picture Midna riding Twilight, even made a picture of it. b.) Princess of Friendship. Maybe that's what she'll be. And she'll probably be just like Cadance before she became ruler of the Crystal Empire, note how neither have the same flowing mane and tail like Celestia and Luna.
So why is Luna so popular? Because people love her or because they hate all other alicorns? She's my favorite alicorn but I have nothing against the wonderful ruler, the Knight of Cerebus slayer, or the Twilight Princess.