Best C compiler for xxxxx processor.....it is always so difficult to say what is the "best". Generally the best one is the one fitting better your knowledge, your application ..... and your budget.
If you're searching for the better ANSI compliance, your selection must falls on a compiler like IAR. Obviously this maybe not so good for you pockets, provided that you have limited budget. In that case you may find a "nearly" ANSI compliance with limited omptimizing options but still decent overall features in the ImageCraft Compilers.
It has been said that ImageCraft Compiler for 68HC11 provides for a code optimization of "only" 4 or maybe 5% less of a better and much more expensive compiler. Unfortunately if you are managing a single-chip application using, let say a 68HC11E9 which has 12K of code, these 4/5% may to do the difference.
Of course, why wouldn't you be able to develop your 68hc11 software using GCC. GCC cross compiler for the hc11 is fully asci compliance. Here is a link where you will find the compiler + examples. The port is for 68hc11 and 68hc12.
I use Imagecraft HC11 with satisfaction from years. It fits my needs, my budget, etc. It's not too good in optimization, but for critical procedures I use asm. :wink: Note that if you need to interface with an ICE for source level debug ICC11 isn't so easy... better going for other (expensive) ones... :?