The options are endless, but important consideration must be made to intended use, where you live and budget.
If your stang is a daily driver but used at the track, you could have a set of all season, and a set for running at the track. Or, go with a performance tire and avoid the snow.
Regarding the letter ratings, these are called "speed ratings". This is related to how much heat the tire can withstand. Use this table below for reference:
P - 93 miles per hour
Q - 99 miles per hour
S - 112 miles per hour
T - 118 miles per hour
U -124 miles per hour
H - 130 miles per hour
V - 149 miles per hour
W - 168 miles per hour
Y -186 miles per hour
Z - 149+
In the case of the mustang, you generally need a W, Y, or a Z rated tire. Snow tires, however, are rated less.
BTW, the stock rims are actually 7.5 x 17 but commonally called 8 inch wide.
For a good all-season tire, I would use this Continental Tire:
http://www.tirerack.com/tires/tires....omCompare1=yes
For a summer only tire, these are GREAT choices for the money:
Kuhmo Ecsta Supra 712:
http://www.tirerack.com/tires/tires....omCompare1=yes
Sumi HTRZ II:
http://www.tirerack.com/tires/tires....omCompare1=yes
Yoko AVS ES 1000
http://www.tirerack.com/tires/tires....omCompare1=yes
A good choice might be to use a perf summer tire and then a all-season tire in the winter is you think snow could be an issue. Of course, there is always the option of a snow tire too if you live where snow is heavy and must drive the Mustang.