I'm interested to see a more concrete power level for Speed Demon, although I think Quicksilver will be faster than him anyways. Originally, Sanders only ran like 150 mph, then he made another formula and took it, which increased his speed, but did it just double his speed? Or perhaps it multiplied (squared) it. So he could be anywhere between 300mph and 22500mph for speed. Although I think it'll probably end up being on the cusp of speed of sound or so.
Northstar would be Quicksilver's closest rival.
And Quicksilver is my very favorite X-men character ever, but Northstar is faster.
QS would give him a good run, but NS in the end would eventually overcome QS with his superior speed and light-emitting capabilities.
So yea. In short, Northstar is Quicksilver's closest rival. No one else really comes anywhere close to either of them.