I would go with Gardevoir.
Starting things off, Wigglytuff is the weakest member of the bunch, in my opinion. It has a tempting 140 base HP, but the rest of its stats are very hard to defend, as it packs atrocious 45 base Defense and Speed, and 50 base Special Defense. Its Attack is also a measly 70 base. Its only other decent stat is Special Attack, at a mediocre base 85 (though it isn't that terrible).
Moving on, I would only really consider Breloom and Gardevoir. This is Sidney's ORAS team:
Mightyena; Ability: Intimidate; Moves: Swagger, Take Down, Sucker Punch, Crunch.
Shiftry; Ability: Chlorophyll; Moves: Fake Out, Feint Attack, Extrasensory; Leaf Blade.
Cacturne; Ability: Sand Veil; Moves: Leech Seed, Payback, Needle Arm, Spiky Shield.
Sharpedo; Ability: Rough Skin; Moves: Crunch, Poison Fang, Aqua Jet; Slash.
Absol; Ability: Super Luck; Moves: Aerial Ace, Night Slash, Psycho Cut, Slash.
Bold + Italics = stuff I'm focusing on and I'm to talk about in a second
Ok, so, Mightyena, Intimidate, Extrasensory, Rough Skin, Poison Fang, Aerial Ace, and Psycho Cut are what I'm going to focus on, as I said.
Mightyena- Mightyena has 70 base Defense and 60 base Special Defense. While these are both terrible, Gardevoir is a Special Attacker and Breloom is a Physical Attacker, so technically Gardevoir will be able to take out Mightyena easier.
Intimidate- Intimidate lowers your Pokemon's Attack, which won't really affect Gardevoir, but it could definitely hinder Breloom's performance, since it is a Physical Attacker.
Extrasensory- Breloom is weak to Shiftry's Extrasensory, so you have to be careful if you were to use Breloom against Shiftry.
Rough Skin- Rough Skin is another thing that Gardevoir, as a Special Attacker (mostly) avoids, but targets Physical Attackers, such as Breloom, taking 1/8 of its maximum HP when it uses a contact move.
Poison Fang- both Gardevoir and Breloom are weak to Poison Fang, but I thought it was worth mentioning.
Aerial Ace- Aerial Ace is the by far most dangerous thing on this list for Breloom, as it has a 4x weakness to Flying.
Psycho Cut- with Super Luck, Psycho Cut will most likely crit, and Breloom is weak to it, while Gardevoir resists Psycho Cut.
So, as you can see, Breloom has a clear disadvantage in these matchups. Really, one of the only things going for it is that it actually resists Dark, plus it has better Defense than Gardevoir and the majority of the moves that Sidney uses are physical. Oh, and its Attack is base 130 and Gardevoir's Special Attack is base 125.
Hope this helps! :)
