Who said this is so simple. If you just want the best, read the reviews and get it. But best lenses have their prices. There are some questions that only you can answer. What do you need the lens for? Is this for a "Pro" use, or just for weekend snaps? How much money can you spend without going hungry or homeless? We can't answer this for you.
One thing I can say though is that I got great bargains by getting a used lens. Wether you buy from a reputable shop or ebay, there will be a huge saving done with a second hand lens. when I first got my D700, I needed a walkaround lens. I thought the 24-70 was way too expensive and the 24-120 on the heavy side. Well, I shopped ebay and got a Tamron 28-200 for 125$. It's not as good as the 24-70, 24-120 or 70-200 Nikkors 2.8, but it served me well at the time. And sometimes, I still will put it on even if I now have the others because it's still my lightest lens to cover FX.
So, take your time, read, take pictures with what you have and then see what you can get.
I forgot that a macro lens is a lens that I would never part with. (just to give you more food for thought)