i just bought a 30-30 and i was wondering what is the best bullet for whitetails a 150 or a 170 grain.

Either one will do the job, there ain't no best. Buy a box of each and see which one your rifle likes best. Different brands shoot different too. There are 130 grain bullets loaded, also.

i dont personally use one, but my dad uses 175's and those work good, i wouldnt go any lighter

Never heard of a 175 for the 30-30. You will find 150 and 170 about everywhere. If shots are 100 yard or less the 150 will work fine. If shots are beyond that the 170 will pack a little more punch. If youur gun like one or the other quite a bit better then stick with that for the difference is slight.

My own experience with a 30-30 for whitetails is a bit limited, but those I got, I did so
with 170gr. ammo. No complaints. I used Remington Core-Lokts.
Some friends use 30-30, and most go with the 170gr. I (and they) thinks the 170gr. give a little better penetration where that's a consideration, and the 150gr. just doesn't make enough difference in trajectory to matter much.
As noted though, either will probably do ok. What shoots best/most accurately in your
rifle? Go with it.

Either one will work fine. I use the 170 grain bullets myself. You need to buy both and see which one your gun shoots best.

I agree, most people use the 170 for deer. It packs more of a punch. The 150's are still good and, IMHO, are a little more accurate. I have run both through my Marlin 30AS and both shoot well. If you go way far beyond 150 yards, the 170's will have more pronounced "bullet drop" then 150's, but then again, you never know.... It all depends on what flavor of 30-30 ammo you're rifle likes.

Butch A.