I'm with banjo on this one - hatha yoga is the most chilled, and the most common one at local sessions. It usually ends in a chill-out meditation session, which will have you feeling relaxed very quickly. Best bet is to speak to the teacher before you go - they will be able to tell you whether their class will cater for your needs.
I've also been to several different Pilates classes in recent years and my current one includes a relaxation session at the end cos the teacher is also a yoga teacher.
Hope you find the relaxation you're looking for! And remember that a bit of gentle exercise, irrespective of the type, is always going to help you beat stress. x