I know I’m a few days late here but, I had 2 60s in 2019 classic and they were priest and warlock, so I’m happy to talk about both with you.
Warlocks are amazing levelers. Honestly maybe even better than hunters (though that is arguable). Only an aoe grinding mage will for sure beat them out, but that’s finicky and I’m not a big fan. Where hunters do have the superior pet, warlocks have the ability to be incredibly efficient with their health and mana. Basically, your health, mana, your pets health and mana, are all like one big resource, especialy if you go affliction.
You can turn your health into mana with life tap, your mana into health via drain life, siphon life, your health into your pets health with health funnel, your pets mana into your own with dark pact. If you manage these well, you’ll never need to stop and eat. You can solo elites no problem, you can take 4 or 5 mobs at a time juggling with fear and sharing aggro between you and your voidwalker. My warlock was undead too,so cannibalize almost completely replaces the need for food altogether (if you’re fighting humanoids/undead).
Demo is also really good, probably even better, I just really like affliction. The thing you really gotta watch out for is over aggroing and taking off your pet when you dont want to.
Priests are also solid levelers. Not as good as locks but still solid. I leveled mine as disc. I did a lot of dungeons but also a lot of questing, didnt find it too bad, certainly better than warror lvling. The trick with priest is to shield yourself, cast a few spells, then wand. Get your spirit tap proc from the kill, regen mana, rinse and repeat. Doing this is… somewat slow, but also very efficient and also will require little drinking.
A fun thing with priests is they can actually take small groups of elite mobs if theyre humanoids by mind controlling them. The trick is to drop the mc last second so you can tag it before they die. Doing this they can take more elites than a warlock, but probably not more regular mobs.
Whatever you choose to play man I hope you have fun, classics a different ball game than retail (I like both!), but still a great game.
Ps: i also reccomend tailoring and enchanting… bags, gear, and wands for yourself