Personally I use the Sigma 150/2.8 or the Nikkor 300/4 with extension ring or converter to photograph large insects.
With the PN11 and the 300/4D AFS I get to 1:2 at 75cm working distance :-)
We agree.
The Sigma 180 f/2.8 is a better lens than the Sigma 150 f/2.8, but they are similar.
Nonetheless, I recently got rid of my Sigma 180 f/2.8 in favor of the Nikon AF-S NIKKOR 300mm f/2.8G ED VR II as a field lens (better stats than any macro lens on the planet + can be converted to 1:2, as you said, by slapping on an extension tube, still retaining excellent stats).
For 1:1, I became intrigued by the Voigtländer 125mm f2.5 Apo-Lanthar, thanks to Michael Erlewine, and am enjoying this for static 1:1 macro shooting, where quick-action isn't called for, but where I can take my time and compose the shot.
For beyond 1:1, I am now reversing some "old school" Nikkor lenses (50mm, 35mm, 28mm, and 20mm), for 1.1x, 1.8x, 2.1x, and 3.4x macro shots, respectively, but these are for tiny subjects that don't move, as the focus distance with reverse-lens shooting is minuscule.