I use QuickTasks from ScaryBear Software (
www.scarybearsoftware.com) - I found it performed better than anything else I'd tried (tAgenda, TaskPlus). They have a Calendar plug-in too, QuickAgenda, which I also use.
Depends on whether you're prepared to pay for something, or would prefer something free. They do offer trial versions though, so you can test it out before committing.