Silent Threat: Reborn is the definitive fan-made Freespace campaign. There are plenty of others, but do yourself a favor and play that one first, because it sets the bar damn high.
I completely disagree - it's one of many strong campaigns that are of a quality, but which differ in their areas of excellence. ST:R's strengths lie in its adherence to the style and quality defined by Volition. Other campaigns may impress you in different areas; Vassago's Dirge makes the most creative use of music and has the most sophisticated sense of visual style. Yet other campaigns excel through technical feats - enhanced AI that dwarfs the original, or tricks of mission design that were never possible in Volition's time. Some depart considerably in narrative style (Derelict, for instance). There's no single best campaign and nothing anywhere near definitive.
If there's one truly astounding strength to the FreeSpace community it's the diversity of campaigns that have been produced. Mind-bending RTS campaigns, capital ship command, dialogue trees - we've done it all.
Make up your own mind about what interests you.