Robin II: The Joker’s Wild miniseries was one of the first comics my sister gave me for my 6th birthday. I credit it with getting me into comics, in that sense of impact.
The Green Lantern: Rebirth miniseries as I’ve detailed elsewhere was very inspiring and affirming for me as I found Hal’s struggles with Parallax to have some applicability to my own substance abuse struggles (my appreciation of the miniseries is detailed here).
Similarly, I started re-reading the 1996 Nightwing series during a pretty dark time in my life. This one quote from Issue #53 (among many others) helped me through that time.