I think President Hinckley put it best: Vote for the candidate who best represents your ideas of good government.
That said, one factor in deciding who to vote for should be electability. It seems silly to vote for the exact match to your political beliefs if the candidate has 0.2% chance of winning.
It's most realistic to support the guy who's near the top and matches your ideals most of the time.
Reagan wasn't great because he was perfect, but because he did most of what he did very well, and for the right reasons.
Does that mean a liberal could be great if he did most of what he did very well, and for (what he believed) were the right reasons?
That is a great observation. I think it has a lot to do with seeing the big picture and having an optimistic vision for the future. While every step one takes or decision one makes may not be the most conservative one at the time, it can be leading in the right direction rather than two steps backward or to the left (if you know what I mean.)