Can I be sued for defamation on youtube?

Well....OK, anyone can be sued for any reason.  The question is rather they could win or if it were declared frivolous.

Defamation is really hard.  Express everything as a personal opinion and they haven't got a chance.

Yes, a person can be sued for defamation, and that includes information posted on You Tube.  Do you know the definition of defamation?  Something I wrote for Yahoo follows.  NO disclaimer protects you from being sued for defamation.  Your odds of winning or losing depend on WHERE you are and your exact language.  (Read the definition and you'll realize why you cannot protect yourself against a defamation lawsuit if, in fact, you are defaming someone) 

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"In general defamation is a false statement to a third person, in print (or images)..Slander is a false statement to a third person in “words.” Gossip falls into this category.  Libel is also defamation if words or images are involved - not spoken.  Printed words or images are necessary in a claim to prove slander and libel..For either definition to be applicable the statement must be LARGELY (not 100%) false.  However, “public disclosure of private facts” is also actionable. This also includes invasion of privacy, intentional infliction of emotional distress, misleading statements - not 100% false but definitely misleading.  Depending on location these can be civil or criminal matters..In order to win a lawsuit the Plaintiff (the person slandered, libeled) has to meet proofs: he/she was injured by the statement (usually financial loss or loss of status), the statement(s) is(are) untrue, if the person who expressed the slander/libel had checked they would have known it was not true. In some cases the Plaintiff must prove that the statement was made to cause intentional harm, that the person who made the statements knew they would cause harm.  There are several ways a person must go about proving that libel has taken place. For example, in the United States, the person must prove that the statement was false, caused harm, and was made without adequate research into the truthfulness of the statement. These steps are for an ordinary citizen."

It is unlikely; this does not mean you cannot be sued; just that if you are sued and fight, you will win.  Disclaimer is important and you have a basis for your opinion. Some basis is important, you cannot just make things up that are derogatory about others.