Sorry to be pedantic about this - but are you saying that liquid yeast is better than dry yeast, or are you saying that the some yeasts are only available as liquid?
It seems to me that most comments in this thread are actually saying the latter, which does not imply the former. That is to say, you should use the yeast strain most appropriate to the beer style you're brewing, and often it will be a liquid.
Which then leads to the point - if the desired yeast is available in both liquid and dry, what do you do? eg US05 is the same strain as WY1056. Which is better?