Chaseability, to me, is a blend of terrain and tornado frequency - not necessarily one or the other alone. Western and northern Ohio, for example, terrain-wise is as good as Illinois and western Kansas, but only gets a worthy setup for tornadoes maybe once a year. I think Oklahoma is certainly near the top when taking all things into consideration.
Illinois is a greatly underrated chasing state. The terrain is not only nearly perfect, but the tornado frequency is on par with the Plains states (take a look at storm data plots). The only thing the Midwest mostly lacks is dryline storms with their attendant 'mothership' structure. Tornado wise, I'd argue that the Midwest (centered on IL) contends for a close second to the Plains. I've been here four years, and I've seen tornadoes every year, mostly within 2 hours of home. And those are just the close-to-home events, there are many more I don't chase upstate due to distance.
Events in the Midwest tend to be scattered throughout the year, as opposed to within a concentrated season. Also, many of our tornado events happen with more subtle setups that aren't synoptically evident and don't manifest themselves until the day of. For this reason, it's not a great 'chase vacation' state like Kansas or Oklahoma, where you can pick two weeks in May and see something. But, it's an ideal place to *live* as a chaser year-round, when you can jump on those subtle 'sleeper' tornado days when they pop up. And we do get our share of big outbreaks, in fact we might have the edge on the Plains when it comes to those! Living in IL, you can get close-to-home tornadoes *plus* Plains states tornado days, which are usually easier to forecast in advance and plan a trip accordingly.
I've always been mystified at Illinois' reputation for weak storms and terrible chasing in a lot of the chase community. I'm not really complaining, as I'm more than happy to continue seeing tornadoes with no other chasers around
