Any level you want to. But limit class, power and equipment selection if you're going to generate at the table.
The ideal situation her is the play the simpler Essentials classes. They power selections are largely done for these character so the only real choices are feats and equipment. If you do some advance work it's pretty easy to nail down feats (or you can just only use the feats published in the essentials books as they are both useful, level/tier independent and a much shorter list than the full line).
Equipment presents a slightly different challenge, however you can either build an equipment table for your level (roll dx on the weapon, armor, neck and random table for starting equipment) and assume whatever mundane items are a given if you start past L1 or 2. Or again you can just limit selections to Essentials equipment only.
Getting a group of characters up and going really quickly seems to be one of the things Essentials does really well pretty much regardless of level. Most classes barely have 3 choices to make at any one level so you should be able to build characters pretty rapidly.
If your players do not want to play Essentials then you have some options.
Do some prep work and either build some roll tables, or just narrow power selections at each level.
Make them generate before the sessions.
Generate for them.
But if you want that sit at the table and roll up a character feel the best way to do it, regardless of level in my opinion will be to go with Essentials. The books are well laid out and easily referenceable and there are not too many choices to make as you level the characters up.
Last thing, and this is from playing some one shots with my group (who are mostly players with <2 yrs RP experience). If you're group is not a group that quickly learns character sheets and power synergies etc etc, then choose lower level one shots, that way the character don't have to process 12-15 options and instead have 5-6 different options/turn (This is fixed somewhat by essentials for certain classes (the basic attack focused ones), but even those have more options at higher levels).