Like the man says, just about any application can handle cartoon rendering as well as more realistic work, the fact is it's the artist that makes an image work or not, it has more to do with technique than which software you use. Owning a lear jet doesn't make you a pilot.
That said, if someone was to push me for which app has the best toon output as well as photoreal output XSI would be my choice. Softimage's toon output is known to be excellent (I have used it in a couple of productions and it's fast and very featured). And XSI is also the leader in rendering quality, the only way to beat it is to get renderman into the pipeline, which is hugely expensive and requires it's own specialized skillset.
Lightwave's render quality is getting pretty slick though, I love their HDR support. But XSI is well ahead of the pack in terms of render quality.
But again, it's easy to make crap in any application, and it's NEVER easy to make gorgeous art, it all comes down to the artist.