Steins;Gate is the best Science Fiction Anime.

I also think it's probably the best Time Travel based story in any medium.  

But I want to clarify what I mean when I say it's the best Sci-Fi Anime.  I mean it's the best Anime at specifically being Science Fiction, the best Anime to recommend to Normie Sci-Fi Nerds as a potential entry point into Anime.

There are Anime I like more that also qualify as Sci-Fi.  

Like for example a show that exists in the same Universe called Robotics;Notes.  That show is quite possibly to me at least better then Steins;Gate at what I generally come to Anime for, but doesn't quite nail it's Sci-Fi premise as well.  But still the difference between those shows on both those scales is an ultimately small matter of degrees, if you like Steins;Gate then Robotics;Notes is probably the next Anime you should give a shot to and vice versa.

However A Certain Scientific Railgun and Code Geass are higher ranked candidates for the title of my favorite Anime franchise period, and are also both technically Sci-Fi.  But they are by no means Hard Sci-Fi, but rather the best Anime equivalents to classic Pulps and Serials.

Gundam Anime (the ones I currently like most are 00 and the Trilogy) are as Mecha War stories better Sci-Fi then Code Geass but ultimately below Code Geass in my opinion for other reasons, and still not as great Sci-Fi as the Semicolon shows.

"Why dedicate a post to this distinction?" you might ask.  Because I know to some Internet Pedantists it might be difficult to wrap their head around making this distinction.  That there can be a difference between a show's use of Sci-Fi being specifically one of the things it's good at, and being merely a plot device or setting to facilitate what the show's real appeal is.

Now there might be a better Sci-Fi Anime then Steins;Gate among what I haven't seen yet, which is still a lot.  But I feel like I've seen enough for my opinion to be worth noting, having completed 535 Anime, and started over 280 I haven't finished.  I can't guess right now what percentage of those qualify as Sci-Fi in some capacity, but it's a significant amount.