Gads, Shane, how to whip up a hailstorm in three easy steps, eh?
Can't wait to see the opinions flare up on this one.
To frame my recommendation for you, I want to accomplish three things with your first taste of twinking: 1) The experience of researching and planning your gear, 2) a bracket that exposes you to the wide spectrum of skill that all brackets feature, and 3) the best use of your time to get you involved right away.
With that, I stand by my recommendations for all starting twinkers: go with the 70-74 bracket. Getting reasonably geared (90%) is ridiculously easy if you plan well, but getting that last 10% will either inspire you or drive you crazy, and 70s are a great place to discover where you land. PuG battlegrounds are meh, and will give you a good baseline comparison of endgame/XP-on vs. XP-off battlegrounds. Meanwhile, RBGs and arenas will push you farther if you decide to go that route. In addition, choosing 70-74 gives you many server and class/spec options. Let's hear more about what classes you currently enjoy in WoW and why, before making a recommendation on class.
Part of me really wanted to recommend 19s for you, because they encapsulate the creativity upon which twinking embarks. Strange and interesting gear choices, weapon swap macros, consumables -- you would get exposure to how the little things make a big impact in twinking. But the lower and midbrackets have balance issues, and that skews the whole spectrum of how PvP works in those brackets. It's not "wrong" or "bad", but I think you'd be happier starting at 70 for your first taste, then trying a low or midbracket twinker later on. Otherwise, consider 19s.
I don't recommend 20-24, because that bracket simply is its own beast -- much different than any other XP-off bracket. The midbrackets (currently 29s and 49s) play on scheduled days of the week, and bring higher expectations of PuGs, including voice chat. While that engenders a neat camaraderie, the midbrackets have a different culture. "Hero" play in a midbracket gets you killed and loses games for your team. If you're not used to team coordination as much, starting with 70 PuGs and then graduating to an RBG will do more for you than plunging into a midbracket that plays 2 or 3 nights a week. 80-84 is fun, but has wider-than-normal gear and skill disparities than most XP-off brackets, combined with exceptionally high burst and some very complex gearing options.
In a nutshell, while I love a lot of different XP-off brackets, 70-74 calls you. 70-74 will make it very easy for you to get started, while still giving you a high ceiling under which to grow.
