What kind of pieces should a strong portfolio contain?

>>3108222
Your strongest work - and reflects the kind of work being done by the person you want to get hired by. You should show skills in composition, color, rendering, and most importantly, creativity. (In other words, no fan art.) If you're applying as a concept artist, your vast collection of lollie images are pretty much inappropriate.
Have between 10-12 pieces, no more. Nobody has time to wade through everything you've done for the last couple of years. If you have anything published, that automatically goes in. Unpublished student work is fine, but show that you've been hired, if you have been.
If you're applying for school, just pick 10 of your strongest works, showing how much experimenting and effort you've done. Don't give them "loomis grinding", show attempts at finished pieces. Show them where you are as an aspiring artist, don't try to fool them that you're better than you are.