Here's my favorite eras of Spidey (from near or during my actual time reading Spider-Man, so anything too before my time will be omitted)
ASM 238 - 251: The original Hobgoblin era. The Hobgoblins afterwards would be various stand-ins. This Hobgoblin saga had a great start, and then went on too long.
ASM 296 - ASM 375: This period gives you David Michelinie's spin on villains like the Scorpion, guest characters like the Hulk and Punisher, Sabretooth, and you get a early 90's Sinister Six arc. This is the era of Venom and Carnage, and these stories were the best Venom and Carnage stories. Every Symbiote-related story afterwards is up for debate. You'll want to stop at least at issue 373, the "Invasion of the Spider-Slayer" arc. Go any further and you risk venturing into the "Robo-Parker parents" stuff, and an entirety of dread called the "Clone Saga".
ASM 29 - 38: MJ's gone, MJ returns, mysticism is added to the story (nay) and Aunt May learn's Peter's secret (yay).
ASM 617 & 625; ASM 634 - 637: 634-637 is Grim Hunt, which I do recommend. 617 and 625 are both stand-alone Rhino stories, but believe me they are good, especially the second one. They are also apart of an arc called "The Gauntlet" which was the road leading down to Grim Hunt, but I honestly hated the stories in the Gauntlet except for those Rhino issues, and the rest of the Gauntlet didn't really lead well into Grim Hunt. As in, you actually can ignore or not read those Gauntlet stories and still be able to follow Grim Hunt as if the two arcs were unrelated.
"One More Day" and "OMIT"
You want to be up-to-date on Spider-Man? You must know that the current status quo of Spider-Man is nothing like what you may remember. So much has changed, and you will be lost unless you read these two arcs that explain how things have changed. These arcs are universally panned by everyone, so I do not recommend them on the quality of the stories, but ONLY because of how necessary it is to know how Spidey's gone from point A to this radically different point B.
ASM 648 +: The current Spider-Man, "Big Time". You have reached your destination.
