Good question.....my analogy would be the NFL. The answer is never available until it is the past tense...in the NFL, you draft what looks like a great quarterback...college all american, etc. and he is a bust. If we could see the future and knew the challenges that will come about that are not apparent at this time, maybe we could make the right decision.
In addition, the stuff you are seeing right now makes a lot of good candidates "unavailable" for running.

Thinking back cant think of a candidate that was close to perfect without allowing actual happenings during the administration to cloud the answer.

I suppose you just have to look at the men/women...what makes them tick...the issues of importance to you now and what you think will be important in the future.

Sorry..long answer and could have been short !