Don't hold your breath. Practically no job candidate has exactly what is needed for the highest level of success on the job. As a hiring manager or HR professional, you are always compromising. Hiring is like dating - if you get close to what you really want and need then you find ways to live with the rest. The key to successful hiring is to know what you are getting so that you can decide if you can live with the other stuff, like it or not.
Using a good workplace assessment can help determine how close you are getting to hiring the perfect candidate. Just know that Mr. or Ms. Right is a dream you had last night - a beautiful but totally unrealistic fantasy. A good assessment absolutely has to be job-specific - what works in one job certainly doesn't work in another. If you are hiring an auditor, you want precision, accuracy and quality above all else. Perhaps you can tolerate some quirkiness in the interpersonal arena if you get the other stuff right. If you are hiring a customer service person, you want a decent level of quality, sure, but your emphasis is likely to be on responsiveness, helpfulness and diplomacy. You might tolerate some gaps in precision if you get the people-part right. So look for an assessment that is highly job specific.
In addition, look for an assessment that will give you the information you need to decide whether you can live with identified gaps. For example, if you learn that Susie Smith has low pressure tolerance and the job for which you are considering her is often high-pressure, think about what, if any, adjustments can be made for Susie to keep her cool, calm and productive? What questions can you ask her in the interview to follow-up on your concern that pressure tolerance might de-rail her success in this job? Use the other information in her profile to help you better understand her triggers so you can make an informed decision about her overall effectiveness. Can you (or someone else on the team) coach her in this area to up her comfort level with the kinds of pressure that this job entails?
Assessments that measure a wide variety of essential and desirable traits, or factors are best. Even better, look for one that includes "killer" tendencies (i.e. de-railers) that can capsize overall performance in a particular job. Don't settle for a personality test that only gives you "types" or broad categories of behavior; these will not give you the detail you need to figure out if you can live with Mr. 80% right, or not. In fact, Ms. 75% right might be a better bet, depending on what the identified gaps are and whether you can live with them, or not. Choose wisely, with good data and be prepared to settle if the right core elements are there and you can support further on-the-job development and learning.