IMO the best way to go is a borosilicate glass bottle (handles frequent scrubbing and cleaning agents without problems) with a simple cap (no straw or sippy, mold/bacteria grows in there and it is impossible to clean out fully) a wide mouth (for easily getting a normal sized bottle brush inside) and with a fabric or rubber sleeve to avoid cracking when it inevitably gets bumped. Even then you'll need to occasionally check under the O-ring gasket in the cap for any bacterial growth; there's nothing like prying the gasket out and finding a couple of colonies of black mold staring back at you (ick) to ruin your day. Yes, it's heavy but most of us aren't using them in a situation where saving every gram counts.