Personally, I prefer to keep a buffer of points, so I'm never at a loss if a skin I want comes on sale, or if I decide to pick up a new set of runes, or if I find a free week champion I like enough to add to my roster. I've made it up over 12k a few times, and I'm currently sitting at about 7800.
There are a few champions I know I'm going to buy(Nautilus, Jarvan IV, Skarner, Diana are all on the list), but I'm waiting for sales on skins that I want for them, so I don't, for example, buy J4 now, then have Abyssal Nautilus come on sale while I can't afford to buy him.
If you're looking for me to name something on that specific list, Nocturne would be my pick. He's a phenomenal jungler, with a strong clear, decent early ganks, strong post-6 ganks, and great mid to late game assassination potential. He can also steal objectives from farther away then any other jungler, with his ult-smite combo, provided you have vision.