Hmm,
Plesk for Windows
Plesk for Linux
If you aren't hosting anything important then DirectAdmin would do on Linux.
Sw-soft(makers of Plesk) are such a well organized and setup company. They are staffed 24/7, ticket system and phone, peace of mind knowing they are a phone call away should anything happen.
Helm on the other hand, which I quite like, don't offer 24/7 support and don't work weekends. For this reason alone they lose out on the Windows spot.
DirectAdmin(although a VERY nice product) has only two people working there. Again they have to lose our purely from a support point of view.
(my personal preference and not in any relation to the company preference)