I don't think there is a 6th lost monument around (5 are more than enough ....)
I would rate the races:
Tier One: E3 (borderline), Gent-Wevelgem, Amstel Gold Race, Flèche Wallonne, San Sebastian (missing in the poll, perhaps borderline like E3)
Tier Two: Het Volk, Strade Bianche (somewhat overrated here *, too early in the season and too little history)
Tier Three: Tre Valle Varesine, Milano - Torino (was long time ago perhaps Tier One), Paris - Tours (same as Milano - Torino)
* I agree though it's the race which is moving up fastest and still some potential in future