My favorite first reaction fics, starring the Weasley twins (gen).
"Identical" by InellFred Weasley
Gen
Summary: "They weren’t identical anymore."
Why I like it: Nothing will never be the same.
"Sleepwalking" by InellGeorge Weasley
Gen
Summary: "No one really understands."
Why I like it: Nothing will ever be the same.
"Quiet Steps" by FlorahartGeorge Weasley, Weasley family
Gen
Summary: "He's not ready to be around happy people yet."
Why I like it: Sometimes it takes a family when someone feels alone.
"Spook" by X-parrotFred & George Weasley
Gen
Summary: "Something of a (re)solution."
Why I like it: Some surprises are bad, some are good, some are just plain hard to deal with. Gred & Forge deal with surprises in their own distinctive way.