Which one... I voted Drayden/Iris.
Why ? It's simple.
The triplets you only battle against the type who have an advantage on your. So, you receive a kind gift... (the monkey you need to beat the gym leader, a little training and you beat his team easily).
Lenora we can find battle type near his gym. And it's not difficult to train a battle pokémon with the Audino...
Burgh if you took the fire type as starter... If you took the water or grass it could be a little harder to beat him. BUT if you train your team on road 4 before fighting him, you could meet a fire type... (a little training is efficient).
Elesa... only I you choose the oshawott starter, you could have some difficult (if you don't take a good pokemon). But with a Maractus, it's really easy.
Clay, a pokemon water or ground is really good but you can also use a fire pokemon. Once his Excadrill is knock out, it's easy to win.
Skyla... it's the easiest gym for me... (Yeah, if you use a electric pokemon it's easiest), but it's not really difficult to win there.
Brycen esay just like burgh. If you have fire starter (or if you pick a fire type on road 4, he's enough train to fight against ice).
The more difficult for me, is to fight against dragon leader. Because, to be efficient, you need a dragon or an ice pokemon. But you only find ice or dragon in previous town. So, it's a little short to train the pokemon before go in front of gym leader.