Sorry, I'm gonna be a hater: I think Poison is such a useless type, especially if not paired with another type.
Its just that, there are so many types it does crap against and "broken-as-shit Steel" types are completely immune against it.
What is it good for? Killing Grass types. But even then, Fire, Flying and Ice are still much better types to have in your team than Poison.
Its more of a "computer type" where characters in the story use them to be a nuisance and poison you. Its not so much as a type for actual players to use competitively. (Except using the move Toxic/Spikes but those don't even do direct damage.)
I'm just sayin'. If a Pokemon could learn Flamethrower and Sludge Bomb and you only had room for one of them, which would you pick?

I know its a Nuzlocke run and you don't have much of a choice, and I assume you like Arbok and Golbat anyway. I think they're pretty cool too.
Like I said, I'm just sayin'. (just bein' annoying)

Oh I know, advantage-wise, Poison isn't that great. It doesn't help that about half of the Grass types out there (and nearly all in Gen I, except for Tangela) are part Poison themselves, completely negating the weakness. And as far as I know, there aren't many Grass types that can resist Fire at all, save for Lileep and Cradily, so that's a far better choice in that regard.

HOWEVER. As Koga will be kind enough to alert you to, especially when you fight him as an E4 member in HG/SS, Poison can be a deadly type in itself. It's one of the worse status conditions to inflict upon something, especially when it's bad poisoning much like Toxic and Poison Fang will do. Not to mention that some Poison types like Swalot, Muk and Koffing are specifically built to wall you while the poisoning slowly whittles down your HP.

And many of the Poison types are kickass outside of their typing to begin with. Moves like Poison Jab and Sludge Bomb on poison types are actually pretty damn powerful, lack of advantages aside. And then there are mons with secondary types like Crobat and Drapion who will show no mercy, Crobat because its Wing Attack/Fly can still fuck you up, and Drapion because it only has one weakness and high stats.

BUT YEAH, Arbok and Golbat are still pretty kickass. C:

I'll argue this. Poison is great defensively. One, they can't be poisoned, which is more handy than you give it credit for. There is nothing more annoying than getting your staller/wall poisoned, you don't have that issue with a good poison type.

As a whole, their defense isn't terrible either. You can crack Wheezing across the face point blank with Iron Head and NOT OK-O it.

A lot of them have FANTASTIC move pools too. Seviper, for example can learn Sludge Bomb and Flamethrower and has a good enough Special Attack to make use of them. Is he going to come in and sweep an EV trained, specially built anti-poison task force of steel types? No. But in game, he'll tear your opponents a new asshole, without even trying.

Plus, this is the type with Crobat, the only Pokemon who almost ALWAYS gets priority on everything. A speedy Crobat with Hypnosis, a good STAB, and Crunch will get you far in the game, and even competitively if you play your cards right.

Poison's major weaknesses? Ground, an amazing offensive attacker, and Psychic, a fragile, almost useless special attacker. Poison almost has an ADVANTAGE over Psychic with all the Bug and Dark combinations out there now. A STAB Crunch or even STAB Silver Wind is going to put the hurt on a Psychic type, if not OKO them. Go ahead, throw a well trained Venomoth at an Alakazam's face and let's see who comes out on top. Ground is a little trickier. Going toe to toe with an offensively built Ground type 'mon is scary though, even for Ice Types. We have our fragile speedster Crobat who's /Flying type gives him immunity from Ground STAB, but I'll be honest, even a STAB Air Slash is gonna be like a Pidgey using gust on a Suicune. My advice? Just don't do it. So you routinely send out your fire types against water types just because? No, I didn't think so. If you do you're being silly, and furthermore you're tempting fate in a Nuzlocke.

And one more thing. Ice is ATROCIOUS as a pure type. They're glass canons. NO deference whatsoever, with okay specials is largely what you're getting with Ice. If it's not Ice/Water, chances are it's gonna get smashed flat. Water provides the bulk, while Ice provides the STAB for Ice Beam you need to take down those pesky Dragon types. Honestly though? A solid Water/Ground with Ice Beam will do you the same amount of good in the long run, the STAB almost isn't worth it.

HOWEVER, it is important to remember that all Pokemon are only as good as their trainers~ I play almost exclusively with mono-type teams, because I enjoy the challenge and fun of working around a particular type's strengths and failings.

This isn't, like, an attack or anything. Just playing the devil's advocate~

Bah, even still, nothing will change my mind about Poisons. Even if they can manage to take a hit, I don't normally use that advantage to set up a strategy. If you can poison them, good, you did your job. What is this Stealth Rock, Swords Dance, Double Team, Baton- Pass nonsense?
I worry less about what I can do to prevent the opponent setting up on me, and worry more if I have a type and speed advantage over them so I can stop them before they set up.
......uh..... off topic, I guess...
I mean, I can understand if Seviper can be good, and I KNOW Crobat can be good, its just no matter what, I still would want a type I feel like would benefit me more. Its all opinion and preference.

As for the Ice types, yeah I agree they aren't that great as a solo type. I was referring more to the choice of moves though. I meant, I'd rather take an Ice Beam over a powerful Poison type attack, because not only can it, as well, take out Grass types, but those really annoying Dragons too, like you said. Ice types are indeed better off paired.
(Just, enough with the Water though. I wanna see new type combos with Ice. A shame there's only new solo Ice Pokemon in gen 5. They added 3 families of Ice types and not one of them were paired. Disregarding a certain one.)

All Pokemon are as good as their trainers... yeah. I rely on luck, and think of all battles as luck. That's why I mostly do bad.
I mean, you might be thinking of a strategy, but at the same time, you have to think about what your opponent might do. Will he attack to finish my Pokemon off? Will he stall? Will he set up something for his next member? Will I be able to pull this move off?
I said before, I am absolutely NOT an expert. But I like to help out anyway I can, and maybe learn from others. (I'm stubborn though, so I may not accept all tips.)

And its cool. No offense taken. We all play differently right?
........*re-reads*
*sigh* Hope I made sense, and didn't say anything stupid.

POISON IS DANGEROUS!

CAPTAIN OBVIOUS