It's really hard to identify what manufacturer was more reliable as it can vary big time from game to game. People generally will say that Bally or Williams are more reliable, but that comes from the fact that they're easier to service because they basically owned the industry through the solid state years and it's very easy to find replacement parts for the machines. As a result, the machines are usually kept running well over times.
However, I own Bally, Williams, Data East, and Stern machines from several different periods of time and I haven't found any to be any more problematic than any other. It's just easiest to find parts for the Bally/Williams machines when needed.