I would have to say mamewah. I have yet to find a front end that is exactly what I'd like, but unless I write my own then I probably won't. Oh, and I think it's windows only, if that matters to you.
Hey, slightly off topic... anyone know of a decent program to edit MAME .ini settings? Other than notepad?

A few of Mamewah's good points, off the top of my head:
- Music in the background of your lists, screensaver, etc.
- Custom lists are relatively easy to do, and there can be a large number of them available to you if you need them.
- screen saver options are nice, especially the random game one. =)
- Custom layouts are easy to do.
- Many resolutions supported.
- Accessable with only a control panel (under ideal circumstances =) )
complaints (as much for minwah's benefit too, so a bit more descriptive):
- Occasionally mamewah will lose it's focus and I have to pull out the mouse to click in it. This seems to happen when mame quits because of a problem with a rom or a non-working rom that I try to play.
- My biggest gripe is setup. There are too many things that could be streamlined with a separate setup app. There is mamewah setup that was created by someone else, but it seems to be a bit buggy here and there. Also the options menu (in my opinion) could be set up a lot more intuitively. For example, some game list options are under the emulator category rather than the game list options.
- I wish the random game option was more accessable.
Hope that helps.