I like Lannan but I don't think he'll ever be a number 1. It's nice to imagine him being our 3rd or 4th starter though when we have at least 2 or 3 pitchers who are better.
Nobody seems to have a consistent definition of what a #1 is, so I have adopted a clear cut statistical ranking as being the way I look at it.
30 teams x 5 starters = 150 pitchers. The top 30 must be #1's, the next 30 #2, etc. I like VORP because it's ballpark and league adjusted, unlike ERA's. Of course, the VORP stat is somewhat arbitrary, as are all stats that involve weightings of different factors, or judgement. So we can argue till the cows come home and it's never resolvable.
But right now, season to date, Lannan just makes the top 30, so he's a #1 (barely) for now. A lower case #1 perhaps, not an "ace", but one of the top 30 starters in the game.