In general, any freeze dryer from an established company whose specifications suit your requirements will suffice. However, some of the major manufacturers — namely, SP Scientific and Labconco — clearly appeal to different buyers.
SP Scientific, who owns VirTis, produce robust, easy-to-maintain, high-capacity freeze dryers best suited for routine lyophilization in industrial settings. While they aren't the most sophisticated systems, they're true workhorses that can handle large sample loads.
Conversely, Labconco manufactures research-oriented dryers. These systems aren't quite as tough as VirTis units, nor are their capacities as large. (Fun fact: the largest Labconco dryer has only an 18 liter condenser, while SP Scientific sells dryers with condensers as large as 50 liters.) However, Labconco systems also have more sophisticated onboard software that make them excellent for recipe development and processing highly sensitive samples.