Tauren are the strongest of all playable races.
http://wow.gamepedia.com/Race
Tauren start with 25 str, orcs start with 23 str . On the alliance the strongest are worgen with 23 str and dwarves with 22 str.
Then there is the "racial modifier" of the base attributes.
http://wow.gamepedia.com/Base_attributes
"Relative to the baseline attributes in the five tables above, characters of the specified race have slightly more or less of each stat. However, adding up all of the relative stats for each race should wind up with a sum of zero, so no race has more stats overall than any other."
Tauren get +5 str, dwarves get +5 str, orcs get +3 str, and worgen get +3 str.
If you add those to the base stats you get Tauren at 30, dwarves at 27, orcs at 26, and worgen at 26. So tauren are the strongest followed by dwarves, then the orcs and worgen tie at third place.