John Scalzi has some interesting commentary on what constitutes "classic" and "good" films in sf over here.
And sometimes I can see where he's coming from. But, I think, where he's coming from is nowhere anyone really cares to go. I mean, Gojira may not be top-shelf entertainment like, say, His Girl Friday (1940) (and don't let's forget the debt it owes to the Beast From 20,000 Fathoms or King Kong (1933)), but I think his opinion that the Emmerich Godzilla remake is a better movie is absurd. Gojira, unlike Emmerich's effort, is at least watchable. How can it possibly stink more?
Other points are valid enough: the science is suspect in Gojira (something not improved upon at all in the Emmerich Godzilla remake--that "It's a Theropoda Allosaurus" line alone isn't going to be forgotten by me anytime soon). And it's certainly true that not a whole lot of great sf films get made, even considering how young both art forms are.
For my part, most classics in the genre are more overlooked than looked at--and this does reveal just how utterly subjective what a classic is. I hold films like Thing From Another World (1951), War of the Worlds (1953), 20,000 Leagues Under the Sea (1954) in much higher regard as Star Wars--and I grew up on Star Wars.
I can accept the fact that classic denotes something that is culturally codified and revered. But I also recognize it's a pretty plastic category, subject to the whims of time, change, social relevance, viewership (number of who rent classic films), and any number of other factors, and is therefore anything classic is going to be anything the viewer regards as, well, classic.
Update: which reminds me. I really need to find a copy of this movie on LaserDisc. (It's a classic!)