I have an older iMac, iMac5,1. It is running OS 10.6.8, and it is running REALLY slow. I’d like to re-install the OS to start over. Best OS for me?

10.6.8 was one of the best OS’s. Your issues seem to be far too little RAM – at least 4GB would be useful, more is better in this case (actually almost every case). Your hard drive might also be full, I believe your machine possibly has a user replaceable drive. You should get at least a 1GB drive, or better still, an SSD – try macsales.com. You should have at least 20GB free space on your drive for adequate performance. I found OSX 10.7 to be a dog, but if you need calendar and contact syncing between Apple devices using Calender and Contacts or Address Book, you’ll need to go to 10.7 or later unfortunately.