What is the best light aircraft, under $100k, for IFR flying? ?

Before we can adequately answer that you need to specify the typical "mission profile". Is the plane going to just be used for training, for relatively short regional flights, or for long trans-continental distance trips? Single or twin engine? Soft or hard IFR? In what part of the world, in what type of weather and flying over terrain of what elevation? How many people do you plan to carry? How fast do you need to go? What's your budget for operating expenses? All these need to be answered if you want to make an intelligent, informed decision. 

OK, you're flying IFR; but why are you flying? If you're just trying to learn IFR, rent. Don't buy. If you're training others, you're going to need at least a two seater. What are you doing up there besides flying IFR (assuming it's not for kicks - which means you scare me). Get a plane that meets those needs. In my experience, most planes can be modified/upgraded to fly IFR for less than a ton of cash. 

You are going to have to buy something used.     You need to really look at the maintenance records to find the best.   A plane from the 1940s that has been well maintained is better than something from the 2000s that has missed some maintenance.