It depends on what period of time is fair to isolate, but I'll plug for 1986 to 1988 (and yes, 1989 live). They were obviously awesome before 1986, but I don't think they were great musicians until 1987 or so (I think Eno said as much), and they still had not made one album that galvanized the masses until Joshua Tree. If you look at how many truly great (not just good) songs they wrote and recorded in the 26-28 months between early '86 and summer/fall 1988, it's pretty damn impressive. This includes all of the Joshua Tree, its many quality B-sides (some clearly on par with the album itself), Rattle & Hum's 9 new songs, and R&H's many good B-sides. It takes them twice that long now to produce one 11-track album.
The only other 2-3 year period that comes close is the one immediately after: say, late 1990 to early 1992. They were certainly going through some special creative processes when recording Achtung Baby, and it was an amazingly fertile time.
But I give the nod to 1986 to 1988 because there were more MORE great tracks produced.