The answer might not be as simple as You think. Word “Best” is kind of subjective on it’s own because it’s hard to say what You have in mind. Being more specific, let’s say: fastest (in rendering), with the highest amount of vRam (to fit bigger scenes), offering best value (performance/$), etc. – would help to give You a proper advise.
But it’s not not only the questions about GPU that matters when You’re trying to make a choice. Environment You’re looking to put Your GPUs in (case, just frame or non at all) is important too. As You can see from article with Sebastian – Value based build for Octane Render difference that cooling makes can be as high as 40%! So when You choose a GPU to buy think of the rest of Your system too.
If You go crazy with vRam but let’s say Your scenes are small (You’re working with optimized models, small textures & low output resolutions) Your hard earned money might be wasted, as buying two lower vRam parts for the same budget might give You a better performance.
Again, PRO range cards costing few times more than their GTX line counterparts will not give better (& in some cases even worse) performance in GPU rendering, but the advantage is that those usually offer higher amount of vRam.
In the end for majority budget is important, as they might look to spend only certain amount of money. Thus a list (of chosen metric) in certain price range might be the information they are looking for. Speaking shortly it’s a lot of different things to consider before choosing a right card for Your needs, so You can finally call it “the Best”.
In 2o15 I’ll try regularly update this topic with most relevant GPU models and put them in context of: performance, latest pricing, availability, cooling design, compatibility (case/airflow), software optimizations (influences the speed for cards based on certain architecture) & all other aspects that might come useful to know when You’re trying to make a decision!
(more to come..)