City of Thieves!
Recently, FF fan Rob Hatton has been running a series of polls to determine which Fighting Fantasy gamebook people think is the best. After a number of rounds, 60+ solo adventures were whittled down to just two - Ian Livingstone's City of Thieves, and Ian Livingstone's Deathtrap Dungeon.
Both books were originally illustrated by Iain McCaig, and after a titanic struggle, which looked to go either way for much of the match, City of Thieves ended up taking a small lead over Deathtrap Dungeon, and managed to hold its rival at bay long enough to take the top spot. The final score was 193 votes to 180.