Don't listen to this dude. FF Spirits Within almost bankrupt Square, which has nothing to do with FF7 or Advent Children.
Advent children was decently received, was not a flop and sold almost 5Mil BluRay copies. The animation might feel a little outdated but the action choreography holds up pretty well, especially the Complete Edition.
Chronologically, 'Before Crisis' comes first, its a mobile game released long back, you can read the summary online or ignore.
Crisis Core comes next, followed by FF7 Original, then Advent Children Film, then Dirge of Cereberus, then there's nothing, SE just abandoned the storyline despite DoC ending teasing future plot.
If you're gonna play FF7R, Crisis Core can be played before as its the prequel but even after is fine since the prequel was released after, even in the case of original FF7 and Crisis Core.
And because FF7R changes things (won't spoil anything), you can ignore everything except Crisis Core.
But its nice to know what the original storyline is if you're interested. In order its, Before Crisis > Crisis Core > FF7 Original > Advent Children > Dirge of Cereberus.