It's tough, I don't remember a striker playing this well in L1 this young (admitting Hazard isn't a stiker here).
He's a guy that can dribble with ease. He seems to glide past defensemen. He's perfectly ambidextrous so defensemen can never cheat on one side.
Anelka could be a comparison, but he's miles better than Anelka was at 18 (though Anelka was a seriously good stiker with Arsenal at that age). Where they could compare is that ability to always make the good control beating the defenseman on that first touch.
Call me crazy, but he could step in with the NT and not look out of place RIGHT NOW.
I guess the best comparison is Neymar in that he's already so good so young and can be a threat anywhere on the forward front. He's less flashy in his dribbles than Neymar, he's more Hazard like in his dribbles (efficient short dribbles).
Over the last 10 years on these boards, I have talked about three comparable players in terms of potential : Hazard, Ben Arfa and M'Vila.
M'Vila crashed his career because of non-football problems, Ben Arfa so far doesn't have the smartness of a champion (but more talent than most of the best players in the world) and Hazard, save for this year, has developped into one of the best offensive players on the planet.
IF (big if) Dembele has the mindset of a champion (too early to say, I have heard some conflicting reports on his intelligence from Rennes officials I know), then the sky is the limit.
Unfortunately, you can judge the football skills of any player, but until we know what they have between the ears....
Look at the Menez/Nasri/Benzema/Ben Arfa generation. Should have dominated the world if the skills were the only factor.