definately registered mail is not safe at all....
i have received empty letters by registered mail
and after close inspection i realized that the
guys at the post office opened the letter and then
resealed it (some times they place tape on it with
USPS stamps to cover the damage done to the
letter).....

and even if you insure the package it's useless in
most cases because sending cash to many countries
is illegal, so you cannot declare that you were
sending money.... I once took a look to the policy
about things not allowed to be sent from US to my
country and i was shocked.... imagine that sending
a simple trading card is illegal.... would you
believe that?

EMS seems good enough as it goes so fast that
there's no much time to deal with it... but better
make the package big (but not heavy) to avoid
suspicious thoughts on the post office people.....

if you have to send few money then the option to
send a plain airmail letter disguissed as a
birthday card or similar works fine....  but i
personally wouldn't use this way to send a lot of
money....

Definately, if possible don't send cash..... try
with IPMO and if that is not available for the
country you want to send the money to then send a
Western Union wire transfer (not money order).
Wire transfers from Western Union are valid
worldwide and even they are a little expensive
they are instantanous (you only have to tell the
other person the transaction number) and also you
won't feel uneasy for sending a high amount of
cash on the mail....