Are you specifically looking at PvP, or also PvE?
PvE Twinking is insane at very low levels. 10-11 for most good Twink Specs, but can be really silly at 20 as well due to a meme trinket (but that’s less based on the class itself).
PvP Twinking, I can’t speak for the high level 60s and such, but if you’re just wanting to crap on other players, you twink at 29, because the F2P 20s get shoved into the Twink Queues (which are segregated from normal leveling queues). So you get queues that actually pop + a mixed bag of players so you get some actual fights and some juggernaut stomps.
Note that All instanced Twink PvP is a separate bracket. Anything you see while not exp-locked is not a twink. You may have seen some weirdness with a 70 Party Syncing to queue with a lower level friend.
PvE 10-11s strong specs:
Fury Warrior [11] or Mistweaver Monk [10] + Lifestealing on weapon (they play exactly the same, Whirlwind and Spinning Crane Kick + Lifestealing scaling silly = OP Damage and Self Healing, also means they are strong before even getting geared out).
Guardian Druid [11], Arcane Mage [10], Holy Priest [10], Elemental Shaman [11], Resto Shaman[10] are all very very strong based on their class alone, while Fury Warrior and Mistweaver Monk will suffer a LOT if they ever nerf Lifestealing at low levels.
PvP 29s strong specs:
MM Hunter has incredible 1 shot potential even against other 29 Twinks, unless it’s a Disc Priest or Druid in Bear Form. Strong in general even if the 1 shot doesn’t happen.
Ele Shaman doesn’t hit quite as hard as MM Hunter, but it has some massive hits regardless and comes with a LOT of self sustain via an overly beefy Earth Shield and heals that will basically full heal if you get the cast off. Also has Burrow as a CD for big damage and immunity. I play this more than my Hunter.
Disc Priest. I don’t play one yet, but they’re always annoying to try and kill, even though I’m fully twinked out. They aren’t much of a threat damage wise but they’ll keep their people alive and you have to actually PvP Normally to get rid of them if they’re geared.
Those are the big ones in my book. I see some Druids and Warlocks that occasionally are a threat. Druids with Starsurge or something (not sure if Resto or Boomkin), Warlocks with Chaos Bolt.
But those first 3 are the big ones IMO.
Gearing for Both:
There are exceptions and alternatives, especially if you are doing 20+, but largely you can just farm WoD Dungeons, as the gear can Warforge into Epic Quality. If you’re in it for PvP the Meme Trinkets aren’t relevant as they don’t function there so I won’t go into detail on that.
Heirlooms are essentially greens as of a few years ago, so if you’re using those, you’re basically baseline. You will be a joke compared to a properly epic’d out Twink. Like half the ilvl, at least in the brackets I’m talking about here.