I suppose any Pokemon I've used in a Nuzlocke, really. Still, I'll have to narrow it down to three.
The first is the Sandile line. This one may be a little unsurprising, given that Krokorok is my favourite Pokemon in general, but in any run where I've used a Krookodile, it's ended up being the best member of my team. The only exception to this was my Y Nuzlocke run, where Digby was outclassed by my Octillery, Yelv, but he was still an immensely helpful teammate. Honestly, any Krookodile I've ever used has both been a reliable team member and a fun one to use. It helps that I love the designs of the entire line.
For the second, let's go back to the Nuzlockes. This one is a specific teammate- that being, Six Stars the Luxray in my Platinum run. I don't really know why, but as I progressed through the Nuzlocke, Six Stars just resonated with me in a way few other Pokemon had before. Heck, even others in the Shinx line bored me before! Still, there's something different about Six Stars. Maybe it's the fact that he was an early catch, maybe it's how reliable he's been in general- I don't know. Weird thing is, I have team members that are more powerful than he is, like Grimm the Scyther, but still, I really love him. I haven't completed that Nuzlocke yet, but I have faith that Six Stars wil make it to the end. Even if he doesn't, I'll be sure to keep him around for the postgame.
And then, of course, there's my beloved Rayquaza, Rayquee. I've had him in my games since Pokemon Emerald, and I'll certainly make sure to transfer him to Moon when the time comes.