I'm looking to get a new gpu for the upcoming games this year. I currently have a GTX 660 and I am looking to upgrade, please suggest me any gpu's that are in this category, AMD or nVidia.
It's been rumored that the 800 series is going to release this September so I'd just wait and see what happens. The new 880 is supposed to be more powerful than the 770, 780, and even the 780Ti I believe and is (rumored) to come with 4GB of VRAM. If you get impatient though then you could settle with a 780 SC 6GB, which should be plenty of VRAM for some of the big open-world games coming out soon.
I think the 800 series will come out within the next three months. How long can you wait? If you can't wait get the GTX 770 or 780 or an R9 280 or 290.
I would go with the 780 6GB and not the 770 just because I own one and it runs the most demanding games amazingly well but on a few games like Crysis 3 it's just a few frames under the 60 fps mark and the 780 would clear it. The 770 is a great card though, especially for only $320-400. The price variance depends on which brand you get, how much VRAM it has, and if it's overclocked or not.
You can try the R9 290 TRI-X OC, it offers very good performance at a great price, but I'm not sure that it'll run Ubisoft games better than the GTX 780 6GB especially Watch Dogs.