Dedicated Macro lens for insect, I recommend getting one with at least 90mm focal length. Most of them are pretty good and close in terms of optical quality. It all comes down to additional features, cost and condition if buy used.
- The newer one have IS/VR/VC/OS to stabilize the image. It help for handheld shots but it increase cost.
- Some of them use internal focus system so that the lens won't extend when shoot at 1:1 ratio. The min. focus of the lens is the distance from the subject to the focal plane (which is either the sensor or film), so when the lens is extended (to shoot at 1:1 ratio), the front element of the lens will be closer to the subject. But that is not the case with some lens with internal focus system. However, those have the internal focus system usually cost more.
- Build quality of the lens may also affect the cost. However, most of them are on the good build quality side especially the higher price one or some of the old lens. There is one non-production macro lens made by Cosina (1:2 ratio but comes with a dioptor too make it 1:1) were made of cheap plastic, but not bad at all as far as optical goes.
If I were you, I will first determine the budget and then find a macro lens that fit that budget. The price range vary from $100 to over $1000.