jay chou.....don't kid me thats lame i mean the new albums..and he's just not a good choice for those who wanna learn mandarin
as for band, i recommand May Day(五月天),SHE,飞儿乐队(FIR),牛奶咖啡(Milk&Coffee)
and singers, i'd like to quote AthenaZheng's answer:
"王力宏(Alexander Wang)张震岳(Chang) 孙燕姿(Stefanie) 王菲(Faye Wong)范玮琪(Fan Fan)"
and add 陈奕迅(eason chan),李宗盛(Jonathan Lee),梁静茹(Fish Leong),张艾嘉(Sylvia Chang),张靓颖(Jane Zhang),范晓萱(Mavis Fan),陈绮贞(Cheer Chen)