What is the best Helpdesk Software solutions.

I’m evaluating different types of Helpdesk Software solutions in the internet but seems to many out there.

Could somebody our there share some of experiences on using helpdesk software solutions? and share thoughts on what do think are the best helpdesk software that i can used.

My basic requirements are:
1. web based.
2. can respond via email.
3. flexible reporting

I appreciate any feedback from this group.

Thanks