What are The factors that need to be considered to decide the best kit to use for an DNA purification ?

1. Type of DNA to be isolated (e.g. genomic vs. plasmid)

2. What you are extacting DNA from (tissue, cultured cells, blood, body fluid, etc).

3. What the DNA is going to be used for (PCR, transfection, sequencing, cloning, etc)

quality  & quantity of dna are important factors that need to be looked at, fragnentation should be avoided after purification

simply by the quality and quantity of DNA the kit can give 

the DNA should not be fragmented after purification by the Kit

The most important factors are:

1) from which source the DNA will be isolated (cells, tissue, yeasts, gram + or gram - bacteria, plants...)

2) for which downstream applications will be used,

3) whether it will be archived for a long time (in this case it is best to use phenol based method) or it will be used for imediate analysys (within few monts). For imediate use comertial column based isolation kits are quite good. 

4) availability of sample and its quantity should be considered

in kit method we find out the DNA in several hours, and if we use kit we don't think so DNA is not in any specie.

It depends of the type of the isolated DNA (genomic or plasmidic) and also the origine of the DNA.

I agree with NIDAL muvarak, Points to be considered about the kit is depending on your source and type of DNA and also its final downstream application.

In general, molecular biology  purpose important  is Purity and yield, if chromosomal DNA for cloning purpose the intergrity of DNA(shouldnt be fragmented). If plasmid DNA  downstream appln is transfection/cell culture it is important to look for endotoxin level.