First, welcome to JF, braindropping!
What options/features are you looking for in a head unit? What brands have you used in the past (familiar with controls/menu structure)? Do you have any plans of external amplification?
I'm a fan of Kenwood's eX
celon series (I run a KDC-X993), but anything from Alpine, Clarion, JVC, Kenwood, Pioneer, or Sony should fit the bill as long as it meets your wants/needs.
If you think there might be any chance of running external amplification, look for a head unit with at least 2 pairs of RCA outputs (front & rear/sub selectable) so that you'll be covered if/when the time comes.