DX4win
DX4win is my everyday logging program. I use it since 5.02 version, more than 10 years. Now it is in 9.04 version. The most valuable feature is the accurate data base. I believe that is the most significant feature a logging program must have.
Let's explain it more. Most of the DXers can recognise the DXCC entity when they listen a callsign. Unforunately there are a lot of exceptions. For example KV4FZ seems to be a USA callsign but the DXCC entity is Virgin Islands (KP2). If the database is not accurate this means an entity less for every award I am chasing. Jim AD1C maintains this database for Dx4win. Jim has a very helpful site also with a lot of tips for dx4win.
Beside this, DX4win has all these features a DX logging program should have. Clusters, support for many awards, LotW support, combined search, etc.