From the usual provenance, the year's best--24 in all--with a happy mix of old and new. Among the former, both Gilberts, Dick Francis, Lawrence Treat, Ngaio Marsh, the omnipresent Asimov, Robert Fish and Henry Slesar--the latter with a man who keeps seeing others with no mouths at ail. The ladies continue on their ascendant: Joyce Harrington who won the MWA short story award in 1972 has two more; Ruth Rendell, admirable always, wins that prize for 1974 with her reprise of a menacing childhood game in ""The Fallen Curtain""; and don't overlook Patricia Highsmith's ""Day of Reckoning"" on a chicken farm where all kinds of horror roosts. . . . One just leads to another and another and another.