According to multiple plat Engineers, Southern Hospitality is the best Wrench. I don't really understand it myself, because I think the Jag's extra build speed eclipses everything else.
It is because the jag is broken. To build a sentry gun, it takes about 0.75 seconds.
A 1.5 second difference is 32%
0.75 seconds is 16%, 0.75 multiplied by 2 = 1.5
16% times 2 = 32%
If it were a 1.5 second difference, it would be 32% construction rate, but it is not.
Personally, I run the southern hospitality, but I may consider running the jag if Valve fixes the weapon.