For milling, 80 grit (preferably 60 and even 36 grit if you have a lot of material you wish to remove quickly). In my experience, the heavier grits create deep grooves that take many passes to remove completely with the successive grits. So, I usually stop milling on the courses grits early to allow for this. Your experience may be different, but I’d be overly cautious at first.
I use 120 grit for generally purpose work.
Actually, I’ve experienced the straight-line grooves with all grits, including 120 where I usually stop to allow removal with successive grits using random orbit sanders. Due to this, I rarely use grits above 120.