Its Fall here, so I'm recommending a pocket flask.
Besides the usual uses a flask makes a nice small water bottle for a jacket pocket - the flat sides make it easier to carry.

This is excactly why I have 3 Nalgene flasks. Able to carry 1 litre of water without great bulk.

I have no objection to a flask for its traditional purposes.

However, some of the 750 ml PET flasks for rum and such are a friendly shape for big pockets. I always found that 1.5 litres was the minimum amount of water when going off pavement. Worth a look.

My flasks are for travelling by bus/train. On the bus, my backpack goes into the cargobay (underneath the cabin) and with me in the cabin comes a small satchel with food and water (the Nalgene flasks) and snacks.

It's a shame that SIGG stopped making the oval designs. Primus also had a couple of oval flasks, but they too have discontinued them. Guess SIGG were the original manufacturer.

I have one of the SIGG oval bottles (well, my girlfriend has it now). Should have bought more of them while they excisted....

There are some jagermeister aluminum flasks on eBay. The black ones hold 500 ml, as I recall. They are similar in shape to the swiss army canteen.