I'm surprised no one has suggested a dervish dance magus yet. At level 5, you can enhance your +1 scimitar to become a +2 keen scimitar (giving it the 15-20 threat range). A friend of mine plays an elf with the kensai archetype and he is getting critical threat all the time.
With spell combat and the Arcane Mark trick, you will be rolling pretty much the same as if you were dual wielding. There is also shocking grasp to really add in punch when you need to (last session, my friend crit on a shocking grasp spellstrike and did 54 damage in a single hit, not too shabby at 5th level).
Plus being dex based and with canny defense he has better AC than the pally in full plate in our party.