Depends what you want....
Quick, not too much trouble and great slick look: Ps21 Carnuba
Longer detailing session with deep shine, tremendous beading: Klasse twins, Zaino
Ive tried and used both.
In Canada we have a short driving season, wait,.... what time is it? oops its over! its actually about 4 months so I pamper the 330ci for those months then at end of season put on the strongest protection possible: many layers of Klasse AIO, SG and then PS21 Carnuba.
But do this: get some waffle-weave drying towels and Microfiber buffing towels and never, never let anyone or anything else touch the car!
That's my 1.6 cents worth!
Good luck!