>>9445911Here's what I'd recommend:
Amazing Spider-Man #149 - if you want to read the very first story Ben's in, and why he matters
What If? Vol. 1 #30 - a fun alt. reality take on what would happen if Ben turned up alive after the events of ASM 149, which seemed like a ridiculous idea at the time
Spectacular Spider-Man Vol. 1 #216 - Ben returns and clashes with Peter, kicking off the '90s Clone Saga
Amazing Spider-Man Vol. 1 #400 - probably the best story out of all the stories in the Clone Saga
The entire first volume of Sensational Spider-Man - the first time Ben officially takes over as Spider-Man
Amazing Spider-Man Vol. 1 #410 - Ben becomes Spider-Carnage, dumb story but great Bagley art
Peter Parker: Spider-Man Vol. 1 #75 - the end of Ben's first run as Spidey
The entire run of Ben Reilly: The Scarlet Spider - pretty fun series written by one of Marvel's best scribes
Amazing Spider-Man Vol. 5 #75 (which is really #876) onwards - the current volume, Ben's officially Spidey again and the book's quite fun (it'll probably revert to the status quo before the title gets renumbered to #900 next year.
Also, if you want to know about the ENTIRE Clone Saga and the thought process (or lack thereof) behind it, you should read the Life of Reilly series of essays (pretty easy to find via Google).