I might be a bit biased toward Firestorm, but I've been using it since the first public beta went live. It has only grown more mature as a software since. For me, it's a great viewer because of its customization and familiarity with certain features that I thought were pretty good out of the v1 generation of viewers while keeping the sleek modern UI of the v3/v4 generation viewers.
That said, after some recent usage with the standard viewer provided by LL, I still feel somewhat lost in the preferences panels there. There are considerably fewer preferences in the LL viewer that makes it difficult to truly customize the experience. For a new user to Second Life, this is actually OK. I'd much rather them get used to how the world works than find themselves overwhelmed by customization options in their viewer. It's easy and quick to learn when you have no experience with any other viewers.
At the end of the day, trying a few different viewers from the currently updated viewer list is always recommended. My needs are not your needs, nor are your needs the needs of someone close to you. Take them all for a test drive and see which car rides smoothest for you.