I imagine for the extreme closeups of flowers a prime would be better. It is just a process of elimination until you hit on the right one. I would bet onto 60mm Micro or the 85 mm to be best at that. Someone with either of those lenses needs to weigh-in.
In general flowers are quite large. The DX sensor is 24x16 mm. So even at 1:1 (about an inch across) a flower is much larger, so longer lenses would do. A macro lense is designed and corrected for closer distances, but unless you have really small object like spiders, ants aphids etc, you do not really need that magnification.
The idea of filling the sensor area with normal lenses was to see how much can be filled up. Most of us hanker for a macro lense, without realizing that what we have in our bag will do most of the time. It is only when you hit the limits of the lense, that another is required. For example
. I have shot bees and moths with the 18-55 kit lense. So what if the insect did not fill up my sensor as a macro would do. I got the shot, used the 24MP to my advantage and got decent 500x500 to 1000x1000 pixel shots. With a dedicated lense I would have got the insect at 4000 x 4000 pixels, but till then what I have is nothing to be sorry of. I do have a 105mm F2.8 AIS macro, but its focus has gone very stiff, so till it is repaired, I am now using it rarely.
. Same is the case with a telephoto. One would be nice but till I get one, my prime and the sensor give me 1000x1000 pixel shots of small birds. The 70-300 non VR lense is there, but the sharpness of the 50mm prime gives much better images than that lense at 200mm+.