Female Pandaren, Belf and Goblin + male Worgen look awesome. Velf is absolutely a must, but I think the best one will be Zandalari Troll! They look sickening af, can't wait to see them in Shadow/Voidform.
Aesthetically: all the Shadowy themed priest gear looks great such as the Absolution set. 100% of all warlock gear lookalikes looks amazing as well, as in the Malefic and Dark Coven. Some mage gear looks great as well like the Firelords set.
Casting wise: It is pleasing to the eye that the Forsake Male cast on the right side of his body similar to a hadoken or kamehameha, pleasing to the eye if you're right handed as well. Even the secondary animation looks great especially when you're doing wasted movement jumping during your GCDs.
Same goes for any caster for Forsaken, a Chaos Bolt or a Pyroblast looks so satisfying.
Lore wise: It's a part of their Forsaken idealism.
For alliance: I'd go Void Elf. Simply because they're some sort of edgy special forces elf race from the void. voidform. etc. I'd probably go gnome for the similar casting animations. I saw a gnome in Voidform, looking like a possessed demonic child casting while floating. Human for aesthetics.
In total I have 4 priests, leveled 2 during the BC days when respeccing costed way too much gold, and I had no money to server xfer, friends would just power level me.
I have a Forsaken Priest obv, Troll Priest named ZabraHex with xmogged into the version that other players see, dwarf male priest sitting idly with his Transcendence themed, and now a leveling Void elf.
I tried race changing my human female Shadow Priest to a void elf one. Ended up refunding the service soon afterwards. First, I prefer the human casting animations. Second, I'm failing to connect to the void elves. Being the product of rectally-derived lore, they almost feel like playing some sort of fanmade mod.
But yeah, overall, I'd say that the Forsaken have that easily. I prefer going male for warlocks and female for priests.