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How much money do you want to spend? As others have mentioned, the "budget" macro lenses do a good job. I started off with a Tamron 90.
Now, my favorites are the 70-180 micro, the Sigma 150, and the Nikon 60 AF-S and 105vr. I'm a macro nut though. I love doing macros and close-ups for personal pleasure. For flowers, the 70-180 is my most used lens in the field. It's the only macro zoom available and is much better and more convenient than trying to use tubes on a zoom.
OTOH, I'm very fond of using a Sigma 100-300 f/4, at 300mm on flowers. Great working distance to be had that way, with rather unique results. If you have a 300 prime, you might want to give that a go, maybe with a short tube, depending on the minimum focus distance of the lens.