Paris-Roubaix is not more of a lottery than any other race. Quite on the contrary, actually. The best are usually in front.
Prestige suggests history. Hence, in my opinion, it should be as followed:
1 Paris Roubaix: Queen of the classic. The one that every legend wanted to have in their palmarès until 1990's specializing and EPO era.
2 Milan Sanremo: Classicissima for the Italians. Great palmarès. Quite a unique race. Legendary 7-win record by Merckx. The race has been stolen by sprinters but only in recent years and I still can't understand how they could manage it.
3 Tour of Lombardy: Perhaps not considered prestigious here in the North because it's in autumn. Hard route now but has changed it too often in its history (the 1985 edition sucked all the way!)
4 Tour of Flanders: Prestigious because related to the myth of the Flandrians. But too easy - mostly in the old days before the Koppenberg was added - to be considered a prestigious classic. The poorest palmarès of the four, whatever Descender might think.
5 Liège Bastogne Liège: hard route and the oldest of all but the first editions had a fairly local field up until the 50's and even then the Arrow or Paris-Brussels had a better field. Liège is a top classic only since the Merckx era, when Paris-Brussels was temp scrapped. In terms of hardness, I'd rank it much higher.