If you have questions about the brushes and techniques of these artists, please check these threads people have already posted before starting a new one:
Ryan Church
http://www.cgtalk.com/showthread.php?t=177756&highlight=ryan+church+brushes
http://www.cgtalk.com/showthread.php?t=197610&highlight=ryan+church+brushes
http://www.cgtalk.com/showthread.php?t=156265&highlight=ryan+church+brushes
http://www.cgtalk.com/showthread.php?t=135955&highlight=ryan+church+brushes
http://www.cgtalk.com/showthread.php?t=120881&highlight=ryan+church+brushes
Craig Mullins
http://www.cgtalk.com/showthread.php?t=218497
http://www.cgtalk.com/showthread.php?t=154991&highlight=craig+mullins
Hyung Tae Kim
http://www.cgtalk.com/showthread.php?t=171346