For a field of flowers:
Zeiss 21mm is nice for the sharpness, color, and micro-contrast.
For single flowers, or small bunches:
Any Macro lens or a TS-E....
A TS-E lets you be much more creative with your camera positioning because you can adjust the plane of focus to cover the flower however you desire. For instance, if you want to shoot a flat faced flower (like a sunflower or daisy) from a 30 degree angle relative to its face with a macro lens you might not be able to get the whole thing in focus even at f/22. So if you want it all in focus you have to shoot from a steeper angle, like 60 degrees. Then since you are at F/22 if there isn't a ton of light you may still need to bump up to more like 80+ degrees, or perpendicular to the face of the flower, which is not at all the shot you may want. With a TS-E you just tilt the plane of focus so it runs along the face of the flower and you can get the whole face in focus at a 30 degree angle and f/5.6 (or lower).
Unfortunately most TS-E's cost an arm and a leg, so start with a good macro, the 60mm macro already mentioned would be great, and a good tripod, which helps you get a narrower aperture without having to bump ISO a ridiculous amount.