First I dash through Galaxy tutorial. Once I'm out of there, I hit P2W for x2 XP, and some powers like Sands of Mu. Run straight to where the Hellions are, well, raising Hell off to the west end of Atlas Park, and drub them en masse; I don't go back to level up til I hit five or six - the level-up Inspiration bonus just fuels my murder rampage.
Then I level up at Back Alley Brawler and street sweep down there while hitting DFB any chance I can. Level to around 14-15.
At that point I can usually tell if a toon is a keeper; if it is, I transfer a billion inf to them for some enhancement pocket money. If not, I shelf them. I don't delete them because maybe I'll wanna try again. Either way, I'm probably not doing any more levelling content - it'll be a combination of alt-account power-levelling and trips to hang out with friends in the MSR murder-bowl.