What size tires will fit my rims?

I have a 2001 BMW X5 4.4i. I just bought a set of 20" rims. I didn't know they were staggered, I was quite disappointed. (2) 20x9 (2) 20x10.5 offset: +18 front offset: +25 rear 5x120. Why do some rims come staggered? What is the best size tires I should buy? I prefer the most height without any susp damage