You can't rely on rain to keep containers adequately watered, so your best bets are hardy succulents, such as Sempervivums, Sedums, Delospermum, etc.

In the ground, you could readily go away and not worry about them, once they're established - in pots, plants always need frequent watering, even when its raining, other than November through to early March, so unless you know someone who can come in and water as necessary, its not going to be possible.

Pelargoniums would possibly be a good bet for summer.
Wouldn't bother for winter.