If someone ask for top sql they from enterprise manager can find some expensive queries. But the result may vary based on the metrics or criteria of expensive. Measurement may vary on,
a) Logical IO's per execution?
b) Physical IO's per execution?
c) Cpu usage?
d) Based on number of parse calls?
e) Elapsed time used?
f) Number of executions?
g) Size consumed in shared pool?
h) Number of child versions found?
i) Based on Wait time?