what is the best lense to use for family/child photos....portrait/group....wedding etc.?
As with any thing...especially artistic endeavors like photography...there is no 'Best'. It depends what you, the artist, wants to do. Once you know that, you can choose the best tool for the job. Then you have to consider if you want the 'best' or the most practical or affordable.
When shooting people, you usually want to avoid too wide of a lens because wide angles tend to accentuate things. A telephoto can compress features but you need more room to work.
Are you looking for the very best quality? Top quality prime (non-zoom) lenses will probably give you the best quality and largest apertures...but they can cost several thousand dollars, each. A top quality zoom will also be expensive but more versatile.
Either way, fast lenses (large maximum aperture) are the way to go when shooting people. For zoom lenses, this means F2.8 or better. For weddings, which often involve low lighting...fast lenses are very important.
What camera do you have and what's your budget?
What ISO is recommended for outdoor photos on a bright day?
Always use the lowest ISO setting possible. The only time I would turn up the ISO is when I need to get a faster shutter speed. (or possibly when I want to use a deeper DOF without sacrificing the shutter speed).