Here's what i'd get.
1.iLife. If your Mac doesn't come bundled with it, i say you get it. It's a great bundle of apps and can prove usefull.
2.iWork. Even my wife started using it instead of ms office a few years ago for everyday work. You should have seen her college classmates when she showed a psentation made in keynote... She was the only Mac user around.
3.MS Office. If you need complete compatibility with documents made on a P(ieceof)C(rap). It works great, i haven't had any hiccups as i have almost everyday on windblows machines.
4.MPlayer OSX Extended and VLC to handle your movies.
5.Messages comes bundled with OSX. It can handle your YM account quite nice. Skype you need to download yourself.
6.MenuMeters is a great way to have important data about the system right on the menubar. You can see cpu usage, ram usage, network usage, and so on.
I guess this would be a minimum of apps i'd have on my Mac. I don't care for other browsers since i have never had problems with Safari. Flash player is a pain in the butt, but there are workarounds for it. I use flashplayer and still will untill it gives me some reason to throw it in the trash.
WARNING! Stay away from the so called "optimization programs" like MacKeeper. It's a piece of crap that does nothing good for you. I'd personally shoot the guys behind it if i had the chance for cheating so many "really stupid" people into buying it. Also try and keep away from antivirus software. There are more chances of you getting Aids from a rusty nail than infecting your Mac. That doesn't mean you gotta be foolish and install every piece of bloatware you can find...