Illinois really only has SFGAm as far as notable coaster parks go, so I'm gonna cast my net out a little further to the 5 states that made up the old Northwest Territory from the frontier days of the American Midwest - Ohio, Indiana, Michigan, Illinois, and Wisconsin (collectively known as "the Great Lakes states" these days).
That brings in a pretty solid collection of other coaster parks (including CP, KI, and HW) that will hopefully make the list a bit more interesting than a mere rundown of my top 10 coasters at my home park of SFGAm (which has A LOT of coasters, but not many truly top-tier ones).
1. Steel Vengeance - CP (my overall #1 anywhere)
2. Maverick - CP
3. Voyage - HW
4. Millennium Force - CP
5. Magnum - CP (my all-time favorite coaster for nostalgic reasons)
6. Beast - KI (mainly here for the night ride bonus)
7. Goliath - SFGAm (wonderful at what it does, but they made it too short)
8. Top Thrill Dragster - CP
9. Diamondback - KI
10. Hades 360 - Mt. O. (yeah, the ops and maintenance are horrendous, but the design is utterly brilliant IMO)
it's been many years since i've visited some of these parks, so here are the notable coasters that i have yet to ride:
KI - Orion & Mystic timbers
HW - Thunderbird
Indiana Beach - Drier Looping (obviously)