That's a really tough question for me to answer. If I were to answer objectively, I'd say that a perfect album has the best mix of instrumentals, lyrics and vocal performances, though I've found that most of the albums I gravitate towards only excel in one of these three areas.
What I value most in any artistic medium is a clear and unique creative vision, either in creating something that's entirely it's own thing (I Trawl the Megahertz). incredibly personal to the artist that made it (Electro-Shock Blues) or communicates ideas/emotions that are either new to me or are articulated in ways far beyond what I can muster (The Lonesome Crowded West). It oftentimes means that my favourite albums are far from the greatest or most innovative of all time, but are perfect to me in their lack of perfection.
In other words, anything that gets me emotional (feel free to call me a fag)