I think that Ferrothorn is the most improved Pokemon in 5th Gen. He didn't even exist in 4th Gen and look at him now; fully usable & the 6th most used OU Pokemon.
But on a serious note, you HAVE to include DW abilities because that's the biggest factor added to 5th Gen aside from 5th Gen Pokemon. Otherwise this thread might as well be called "Most Useful 5th Gen Moves + Rotom-W." Why even bother listing the Pokemon at this point.
Scald, Volt Switch, Shell Smash, Psyshock, Quiver/Coil/Shift Gear, V-Create, Dragon Tail, & Wild Charge /thread
I'd say around 75% of the rise & falls of Pokemon this Gen are from new abilities & new Pokemon and not particularly new moves. Outside of that, not a lot of Pokemon particularly improved outside of Rotom-W. (Which is very borderline since it was already "the shit" in 4th Gen)
Let's see, Magnezone got more useful due to Ferrothorn. Celebi got Nasty Plot. Lucario got Nast-...well Special Lucario sucks, we all know that. Eviolite made some Pokemon better but that's a giant can of worms. Smeargle & Gore-Tail (They're interchangeable) got SmashPass. Rain helps Pokemon like-WHOOPS no DW Abilities~.
This topic is pretty thin without DW abilities