Large nipples or large tits?
Personally, I can't stand the massive burger-type nipples. No sir, big wide flat ones are no good for me. It's all about the chunkiness. You want a nice prominent nipple that swells nicely to a chunky nub that you could grab between finger and thumb. A moderately-sized areola that goes all tight, stiff and erect too. I like to see the whole thing spring to life, not just the tip!
But regardless of all that, natural is the key. They've gotta sag just the right amount, hang properly when she bends over, and sway pendulously when she's getting boned. If they don't look right or move right, it's a total turn-off.