Best gun powder for reloading .308 winchester model 70 bullets?

Perhaps your questions are better served in the re-loading section?
"Having no idea what powder to buy" suggests you may want to invest in a reputable reloading manual rather than relying on the kindness of strangers on the internets. I like Sierra and Hornady myself.
 
Yep, buy a couple books. Never, ever, solely trust reloading information of the internet. Most books will give a short bit of important history (the .260 Remington load is relatively new and doesn't require a diminished load for safety in older firearms". They will also publish COAL's and most accurate load data. Use this forum fine tune what the books say. I usually buy a book a year, it keeps up with modern cartridges and confirms whats already been published. I keep a small folder for each caliber with load information and ballistic info for quick reference.