I recommend Star-RC sofware.Because i know it can combind with raphael or raphael-nes(quicksim) to generator RC equeation to extract RC value.raphael and raphael-nes is standard RC-extractor in industry and all LPE tool will compare with them to calibration.So Star-RC accuracy is more than other LPE tools.
If you want to extract a small block you can use xcalibre to do it.
If you want to exract a full chip or big block,i suggest you need use calibre-xrc to do it.
Because xcalibre speed is too slow.
above focus on Mentor extraction tool.
If you want to do post-sim for top down design flow,you will need extract parasitic for a full chip(gate level) to generate a dspf file and use delay calculator(like as prime time) to generate a sdf file and do post-sim.
If you want to do post-sim for bottom up design flow,you will need extract parasitic for a full chip(transistor level) to generate lump C and so on..... and to do post-sim.
Could you tell me your design flow ? If you need to do post-sim,i think i want parasitic extraction.