Which apps are least useful? I've no idea only you can answer that.
For starters, which apps don't you use? Apps like Garageband and iMovie can have several gigabytes worth of files and samples and stuff so they might be a good place to start. Of course, if you decide you fancy a dabble with them in the future you can always reinstall them off the install disks that came with your Mac.
It might also be worthwhile looking at an app like
Delocalizer it'll remove any language localisations you don't need on your Mac. It's surprising how much space the various localisations take up, especially on a laptop where hard drive space is at a bit of a premium.