I can't say for all brackets, but I'm rocking it at 20-29! Especially when it comes to world pvp.
I hang around in Stonetalon Mountains almost everyday, battling Horde players. I've been doing this for two months already and it never gets old, mostly because there are always players at any time of the day. If I opt for tougher challenge, I go to Hillsbrad.
On the other hand, battlegrounds are a wildcard. There will be awesome and ****ty streaks, and you can't predict it. Queues can be really slow during weekdays (keeping in mind multiple rollbacks) and some fights can be one sided where the other team gives up too easily. Kinda sad that I can't say any distinct positive things about BGs in this bracket.
In spit of my enjoyment, I have met very few 20-29 twinks and those I've met often go in their own way. That said, I notice that the the following brackets are the most active: 10-19 (ESPECIALLY THIS), 40-49, and 70-79.
I am the GM of the ally 29 & 39 twink guild <Primary Target>
29s are really active. I'd suggest making one and i can help with gear and runs. We have 4 gbank tabs of gear.
39s were really active before wotlk was released - since then it's calmed down. But its only a matter of time before it picks up again once people hit 80 and focus on twinks / alts.
So yea, make a 29 and pst somebody in our guild for an invite. anyone is welcome and gear / runs available for all :)