If you're like me the default Unit frames leave a... bit... to be desired. Why have plain bars with too little info when you can have interact lines, threat, reputation, defecits, class colors, and 3D portraits!
There's a lot of choices when it comes to Unit frames. Some are pretty easy to get going, others... not quite.
It may be a good choice to start with something simple like X-Perl . X-Perl has a lot of options for customization, raid support & frames, and works well right out of the box. My only issue with X-Perl (and a few others) is the lack of visual changes. If you like an almost default Unit frame with lots of usable info, X-Perl may be for you ! There is also Perl Classic which is a more basic version with a much simpler look and feel. My only complaint was that, in my tests, Perl Classic actually takes a bit more memory than X-Perl, so don't think that less information is necessarily lighter on the CPU.
Now if you want simplicity, you can't get much simpler than Sage . There's a few pieces of info you can enable / disable, but you probably won't do anything more than move them where you want.
If you happen to be the text based type of player who just likes the facts, Satrina is a text only Unit Frame. You can adjust size / position, but that's about it.
Now for the big guns ! PitBull has just about anything you could possibly want ! 3D portraits, built in threat bar, and more customization than you'd probably use. The topper to all this is the visual "interact lines". A simple idea, but can be crazy depending on your placement of the different frames. Any time a player attacks / heals another player a tiny red / green line connects between the 2 players. I had my party and raid bars on opposite sides of each other and the green lines from healing were going nuts from side to side. PitBull is an awesome Unit frame addOn, the only downside is memory usage.
Last but not least, and the WTFBBQ pick, Ag Unit Frames . To me AgUf is a happy medium between info, looks, and customization. You can adjust just about anything and with the addition of AgUf Layouts , there's a layout for just about anyone.
I hope this gives you an idea of all the possibilities for your Unit frames and all the great layouts that can be built around a good looking, usable Unit frame !
Here's the screen shots
For the number crunchers, here's my base memory usage for each addOn in this review:
Ag Unit Frames - 2.4m
Perl Classic - 2.1m
PitBull - 7.5m
Sage - 550k
Satrina - 350k
X-Perl - 1.8m