if we were to exclude superman its pietro. even tho his character has gotten up grades and downgrades over the years he's still pretty quick. back in the 80s he was listed as 175mph, in the early-mid 90s 175-300. then in the late 90s just before the ultimate universe was created and all the other resurrections and new creations of Marvel(Uncanny, NewXmen Avengers, Xtreme-Xmen and Astonishing Xmen) he got some chemcial enchancemnt from the High Evolutionary(isotope E) that made his upper limits of speed unkown(marvel handbook) yeah leave up to those bastards at marvel to be all cryptic and mysterrious.
now i think his top speed is somewhere between mach10-15, but his cruising speed is somewhat lower than that. for in the final battle when magneto was taking over new york and killing everybdoy he ran across the ocean in a few mintues, also in Marvel 1602 he yet again ran across the ocean from one country to another, and in Avengers Disassembled when wanda just went berserk and was tearing up everybody who has ever been a member of the avengers....ever. he wasnt there(a magical doppelganger was but he was vactioning somewhere), but when he heard the news he ran from hawaii i think it was(not sure) and he commented that he just got the news. then when they told him what happened, he just left all emotional and shit and by the time somebody tried to call him back iron man was like .."he's no longer in the city." so if he can get out of an entire city b4 someone can get a sentence off he must be pretty quick.
Northstar however, is no slouch he is listed as having hyper sonic speed.hyper sonic by definition is mach 5 and over so at the very least he's mach 5. tho ive seen him move and leave after images so at the end of the day current pietro vs him would result in pietro by a little bit