I'm just going to go with difficulty since every Gym Leader's personality would take a while. This is only accounting for my personal playthrough witha team consisting of Emboar, Lilligant, Carracosta, Galvantula, Bisharp and Braviary.
Cilan/Chili/Cress: 4/10
Suitably challenging for the start of the game but I was disappointed in how simple it was made by the gift monkey.
Lenora: 5/10
Wow what a bump up. Normal type Leaders always put up a fight. However she was not too bad for me because Petilil took down Herdier and I sacrificed her against Watchog's Retaliate to bring in my fresh Pignite. Lenora was stupid and spammed Crunch so I won quite easily.
Burgh: 2/10
Swept with Flame Charge. Nuff said.
Elesa: 10/10
Oh dear god the memories... My team at this point consisted of Petilil, Tirtouga and Pignite. ALL of whom had a weakness to Emolga. Even worse was that the AI was playing mind games with me and zigging when I zagged. If I tried to turn the tables with tactics and prediction Elesa would go for the simple option and counter it. Probably my most difficult Gym battle ever.
Clay: 8/10
A surprisingly high rating for Clay, I know. But yet again it came down to my team. Tirtouga and Pignite had easily exploited weaknesses and my Petilil was still unevolved and unable to take Excadrill alone. Eventually I lucked through thanks to Tirtouga's Quick Claw.
Skyla: 1/10
Oh God she was a joke. My Galvantula swept her. Whose bright idea was it to put the Flying Gym Leader after an Electric cave? Seriously? And even worse was her signature Pokemon was a WATER and Flying type. Poor Skyla drew the short straw on Gym placement.
Brycen: 3/10
An improvement on Skyla but still very ineffective. Emboar destroyed him. The only reason he got a slightly higher rating was because Cryogonal put up a fight.
Iris (played White): 7/10
A bit of a letdown really. When I first saw her team layout I was worried and the Gym Trainers themselves were surprisingly tough so I was expecting a lot of her. I was challenged but still unimpressed. Her team would be a lot better in variation and challenge if Fraxure was replaced by a Zweilous.
And now the E4, including the two Plasma heads. My team at this point was Rufflet, Pawniard, Lilligant, Carracosta, Galvantula and Emboar, all level 48.
Caitlin: 6/10
When I saw the team layout of all the Elite 4 members I was afraid. Those were some tough Pokemon I was going up against. But Caitlin was... lacklustre. Reuniclus was just too slow to land a hit, Musharna took a while to kill but didn't cause too much damage, Sigilyph fell instantly to Galvantula and Gothitelle just spammed Calm Mind. Pawniard and Galvantula were all I needed for this fight.
Shauntal: 9/10
Most of Shauntal's team were fairly easy. Cofagrigus was walled by Pawniard, Jellicent fell to Galvantula, Lilligant ended Golurk and..... I was swept by Chandelure. Damn her for covering her weaknesses! This marked the first and only time I've lost to an Elite 4 member since Drake in Ruby. Yet again Carracosta's Quick Claw pulled through though.
Grimsley: 5/10
Yes, even easier than Caitlin. The only Pokemon who put up any kind of a fight was Scrafty and one Pokemon does not a team make. Seriously, Liepard? Why does Game Freak hate Zweilous so much? Bisharp does have mammoth attack and a good movepool but it had a glaring weakness easily exploited by its poor speed. Same with Krookadile.
Marshal: 8/10
Throh? No problemo. Sawk? STUPID STURDY Y U NO DIE!!! But he was revenge killed. Mienshao missed Hi Jump Kick and I lol'd and KO'd it. Conkeldurr was extremely powerful and bulky but by this stage it was 5 vs 1 and collective effort took down the beast.
N: 4/10
Once you get past Reshiram which I did without too much trouble, N's team is rather weak. Vanilluxe is easy, Carracosta is easy, Klinklang is easy. Zoroark signed its own death warrant when it came out as Klinklang against my Emboar. Archeops put up a fight but it wasn't enough.
Ghetsis: 8/10
And here we are. Mr Harmonia himself. Ghetsis has a much tougher team than N, and he doesn't even abuse a legend! Though admittedly he hacks. But still. Cofagrigus just can't fight Pawniard, Bouffalant is tough and bulky but eventually fell. Both Seismitoad and Bisharp have glaring weaknesses. Next up is Eelektross. A toughie but it fell to my Rufflet of all things. And finally... Hydreigon. I had saved my Galvantula throughout the battle as my only hope to take on the hacked dragon with his higher speed and type advantage. Unfortunately Signal Beam did only half. But... it was confused! The first time Signal Beam confuses and its on Hydreigon. Hydreigon hits itself and I win.
Galvantula is super awesome.