Best complete bmx is too general a question. What's your budget? You want a street/park or dirt bike? Are you planning on buying from a shop or mailorder? I don't claim to be an expert, but I have bought several bmx bikes over the years (I've got 2 boys and for me). You need to figure out what you want or don't want from the bike. Start with the frame...all chomoly or partial steel is ok? Me, I prefer all chomoly (incl fork). Do you want the gyro, pegs, etc? My boys primarily ride dirt, so we don't bother buying a bike with that crap. Running out of room here, but you can be sure 2010 is a great year for completes. Check out the WTP Envy, Verde Luxe (comes with Twombolt cranks!), the Fit sig completes are pretty sweet, Kink makes a great complete. I have to shop online, prices here in Holland are ridiculous so I haven't even researched Mirraco, for example.
Good luck.