It was first according to PG's knowledge, which is incomplete. Amazon.com was incorporated in 1994, and sold its first book online in July 1995 [1], a month before Viaweb. EBay launched in Sept 1995 [2], a month after Viaweb, and so was probably independently under development. Geocities was active and allowing user-contributed content by July 1995 [3], a month before. WebCrawler was the first general-purpose Internet search engine, and launched in April 1994 [4].
[1] http://en.wikipedia.org/wiki/Amazon.com
[2] http://en.wikipedia.org/wiki/EBay#History