I'm going to share my knowledge about wigs,
but it's limited to synthetic fibers,
since that's all I own.
The best quality fiber is Kanekalon.
I've known wigs that melt if worn outside in hot temperatures,
but kanekalon is very heat resistant and doesn't do this (just don't use blow dryers or curlers or any other heated tools on them,
as the heat is too concentrated for the fiber to handle).
It's also the most realistic with a nice shine and is the easiest to brush out should it tangle.
If you want a highly realistic wig,
then you should look for one with a skin top.
This means that,
where the part is supposed to be,
there'll be a strip of plastic that looks like scalp.
If you want something a little less realistic,
then avoid the skin top.
Sometimes,
the alternatives to skin tops are called packing or ratting tops and have parts covered in short,
curly hairs.
One of my skin top wigs:
http://www.cosplay.com/i/costumes/200/210994.jpg
A packing top wig in a similar style (it's also not Kanekalon):
http://www.cosplay.com/photo/1842582/
When looking for a blonde wig,
make sure it's a true blonde and not yellow.
Yellow wigs always,
without fail,
look really fake,
not just because of the color but I've also noticed that they reflect light differently and so look plasticy.
Finally,
I recommend a wig no longer than to your shoulder blades.
The longer the wig,
the more likely that it'll tangle horribly.
You can help prevent this with a little wig spray,
but still.
Long wigs are known to tangle and be a pain to brush out,
no matter what they're made of.