I really like them all.
Dark Iron Dwarves are both aesthetically and thematically great. But may be should've been just customization option.
Void Elves are...something half assed lorewise, created from thin air, just to not give us high elves. But, hey it is something.
Kul Tiran are long needed Burly option for Alliance.
Lightforged are probably the weakeast, since they could've been just appearance option for draenei.
Nightborne...should've been neutral race, ffs. Also great heritage armor.
Highmountain & Mag'Har - see Lightforged.
Zandalari - for first time in 15 years and countless alts...i made a troll. Speaks volumes.
(And pretending that mechagnomes and vulpera don't exist.)