It's an interesting question, really. If you are simply talking about on-field rivalries, then I suppose the obvious ones are the Dragons and, historically, Manly.
However, these days, I tend to take a bigger picture approach. Which Clubs are the bigger threat to our existence? Ie; who do we need to fight off for long term survival?
In this respect, I have no issue with Manly. They represent the opposite side of the coin to us, I think. Northern beaches, suburban home ground.
The teams we really have to see off are obviously the Dragons and the Wests Tigers. They are competing for pretty much what is similar territory in the south and south-west. The financial collapse of one or both of these would work well in our favour. Personally, I'd like to see the Dragons relocated to Illawarra, which may not be out of the question.
I also believe the Rabbitohs present somewhat of a problems for us, in as much as they steal a lot of support in our local area. Whilst they are owned by a couple of multi-millionaires however, we can't do much about it.
So yeah, if you want to know which teams I really consider the enemy, it's Dragons, Tigers and Bunnies in that order. I particularly hate the Bunnies ATM, a club I never really had an issue with.
That is, until they basically bought a comp and have become an all consuming behemoth that every man and his bandwagon mate wants to jump on. They are the darlings of the NRL and all Rugby League media.
They basically reinforce the fact that there is no room for the smaller community Clubs in the NRL these days.