These days there's almost no such thing as a "senior base" compared to not that long ago. For a while it took years to hold the plug in SEA, LAX and even MSP. ATL was even hard to get for years in some narrow body categories.

These days "junior" means can you get it as a new hire or at least within the first year?

Short answer? Yes NYC is the "most junior" but new hires can and sometimes do get almost every base depending on when they're hired either first assignment or within a year. Always within 2 as far as I'm aware.