There is no 'best' bat.
it all depends on the 'level'...a high level 12U team/player may use the same brand and model as a college team/player.
composite bats are better than aluminum.
a good weight for 12U is 18oz.
An end loaded bat should only be chosen for a 'power hitter' other wise you'll want the bat to be balanced.
I really like the Easton composite bats such as the synergy and the stealth (which has a connection peice). I also really like the TPS Catalyst...however these bats run in the $250-300 range which is obviously an obnoxious price to pay for a bat that is going to be outgrown quickly.
The Loisville Zephyr looks like a decent bat and has a -12 drop so it will be long and light. It is also composite making a lower end version of the Catalyst. However that bat is $180 which would be good for a HIGH Level 12U player
If the player in mind is playing in a rec league and has little experience it is unnecessary to spend any more than $50 for a bat.
www.softball.com has all of the high end bats for sale and are listed by 'recomended' and 'top sellers'
good luck :)