I would also say 2012, which was a total and complete waste of Spider-man's 50th anniversary (Alpha, anyone?). Seriously, it's like they couldn't bring themselves to do any worthwhile stories for the 50th. (I think the only one was Stern's one-off issue of PP:SM) 2012 also culminated in ASM #700, quite simply the worst of the centennial issues yet. 2013 was lame with the Superior stuff, and is also itself a big contender for Spider-man's worst year along with 1999 and ASM Vol. 2.
But I have to go with 1995, like the last time there was this thread. The 90's Clone Saga stuff, while slightly compelling at first, just simply got ridiculous after ASM #400. It was also the year the titles all turned to 'Scarlet Spider' for two months, with some really loud and painful pink-colored graffix all over the covers and terrible plots involving cyberspace.