Let’s take a look then, shall we? Reagan is hard, what with being dead and all and largely pre-dating the internet – I’d have to do a whole bunch of work to collect data on his positions on various issues. So instead, I used the centriest* of the GOP candidates, Kasich. Here’s Clinton vs. Kasich.
And here’s a side-by-side of Romney, Trump, Clinton, and Sanders.
I’d say there’s a lot more separating Clinton and any of the GOP candidates than there is her and Sanders. And, of course, there’s the Clinton-Sanders voting record comparison, where they voted the same way on 93% of the issues. You could certainly argue that some of those issues were extremely significant – the NYT has a good analysis piece – but can you honestly state that there would be anywhere near as much similarity between Clinton and Rubio? (never mind Clinton and Santorum…)
*“centriest” because “most centrist” is boring.