This quote means that Metal Gear was both the biggest success and biggest failure of the speaker. This could mean a number of things, such as that the speaker had to put in a great deal of effort to make this game a success but ultimately it did not do as well as expected; or that the speaker achieved a great success with the game but there were still many aspects that could have been improved upon and thus it could be considered a failure in some sense. Ultimately, this quote means that the speaker sees his work on the game, the successes and the failures, to be a mixed result in terms of outcome.