Best OS X for late 2009 MacBook
You basically have two options:


If you don't want to open up machine then install Snow Leopard as leanest of them all. It will be fast though few resource hungry apps or web pages can bog it down. 
Another minus of the path is that Snow Leopard being released July 25, 2011 is going to be unsupported soon if not already. At least latest NTP patch doesn't apply to it.
If you do to open it. And it is not that hard really with this machine see instruction. Then I would say add 8GB while you are there. It supports up to 2x4GB SO-DIMM DDR3-1066 modules(choose big names like Hynix, Samsung, Crucial for compatibility). Regarding OS X choice it is either 10.9 as more polished or 10.10 as the latest. Clearly 10.9 behaves a lot more solid on the machine. But if you need 10.10 it will also be fine. Safari is definitely snappier on it.


Regarding 10.7 Lion. It just doesn't make sense. It is no faster than Mavericks on 2GB but it is definitely slower than 10.6 Snow Leopard. 

I put 8GB RAM and 128GB SSD in place of Superdrive in same machine recently, installed 10.10 and it will be fine for few more years for light to medium tasks. Good luck