Personally i find 20-24 bracket fun, if you go f2p 20 twink i think Aerie peak would be the place to be on US. Dont know if theres any realms for 24 thats the hotspot or not, if you dig around you can probably find something.
20-24 bracket is fun but at the same time its alot like old school xp off....you deal with alot of people who dont know what there doing and you often find yourself carrying a whole team. The games I have played there was no coordination.
19 bracket is also very fun and I would suggest anyone looking for a active bracket that is easy to gear for that might be the place for you. There are very skilled people in the 19 bracket and the games can get very competitive. But also the 19 bracket has its low times where people do afk out alot and there is farming.
39 bracket is probably the most balanced bracket I have ever played in other than Prot Warriors. The game nights Wednesdays and Fridays at 8pm est time you get into games where players actually coordinate and play the "game" and not run around mid. Not only that but an amazingly fun group of people who are more than willing to play for the game rolling classes that one side or the other actually needs and will help you do the same. They are always great and willing to help you grow and become part of the group. If your looking to do bgs on a scheduled nights so you can focus on other things for the other nights of the week I would defiantly recommend 39s.
20-24 bracket holds the highest twinking activity these days. The majority of the bracket plays level 20/trial account twinks, but you can also go old sk00l twink badass and make a 24 to stomp the poor lowbies. Depends what you're after, but 20-24 means you can play underpowered and overpowered classes at your choosing. p.s Hpala Spriests and Resto Druids are god mode classes. Alexstrasza (sp) has one of the highest population of 24's in US. Good luck whatever you choose