The best way choose the best adjustable dumbbells is to figure out your own goals. If you want to get huge you will surely need a set that also gets huge but if you are just looking for consistent exercise then maybe consider a smaller set. I will say however that I believe the bowflex selecttech dumbbells are the best selling.
If you really plan to purchase adjustable dumbbells you will probably want to read some reviews to find out which set has the features that you desire. I found a pretty good resource for unbiased reviews here: http://thebestadjustabledumbbells.org/
and you can also consider reading the ratings on amazon or from the manufactures directly.