Out of the all common separators that you can use in the title tag or within your URL the one that you do not want to use is the Underscore. So, commas, pipes, dashes, and other separators are good to use.
The problem with the underscore is that Google treats the word before and after Underscore as one complete word. I.E., Van_halen is seen as one complete word, as apposed to Van-Halen, van is seen as one word, and halen is seen as the second word. So when using the Dash (–) between words Google will see this the worst before and after as separate words
Now if you have a webpage that does have underscore within the URL. Google recommends keeping it as is. If you did want to make a change, you can change the underscore to a dash and on the old URL place a 301 redirect going to the new URL. You will want to note that when using a 301 redirect, past research has shown that these redirects pass on 90-99% of the link juice. So, not all of the link juice will pass on to the new page. For all new pages you create, use the dash. Just avoid the underscore!