Hardest? The one I'm working on right now... which isn't necessarily
hard, just boring and time consuming.
A 2-tier client/server app in VB.NET with SQL Server support. It's a distributed app for my dork roomies and their RPGs. We have a LAN, so they want an distributed app to completely eliminate the paper and pencil aspect of their roleplaying games.
So... functional player sheets and dice rollers on teh clients, with the server having full access to the info and the ability to change info (the server is the GM's computer, obv.)... basic interface is done. I have the clients talking to server, and server talking to DB.
Once I got basic functionality done, and communications between client-server-db, I started fooling with making little openGL animations for 3-d dice rolling, hoping to stick them in a box somewhere in a VB window.
It's a huge, uncommented, hog of a program.. and damn messy. I think i may give up soon. Just getting the thing talking to it's different components took out my oomph. i haven't even begun to create the business rules, or built the huge multi-table relational database.
Blah, programming is hard, yo.