So you've chosen a shape, but have you considered the composition of your bottle? We have tons of different plastics to choose from, but each has their advantage. Most of these can't be filled at over 160F, but we aren't going to be bottling anything that warm, are we?
HDPE: High density polyethylene - Good for everything, unless you're using a ton of essential oils
HDPE usually comes as a translucent, flexible bottle that is compatible with just about everything. (Look right - the frosted or the completely white bottles tend to be HDPE). Except things with a lot of essential oils. If you've ever put something high in essential oils - 1% in a lotion is just fine - you'll see it start to buckle and warp. Not a good idea. And don't put anything over 160F into this bottle! You can squeeze and drop it without creating huge dents.
When you find the white bottles that I tend to think of as storage bottles, those are HDPE as well. Great for shampoos, conditioners, and the like.
PET: Polyethylene terephthalate - Good for products containing a lot of alcohol or essential oils, as well as pretty much everything else.
You'll recognize this one from pop bottles. They work well for alcohols and essential oils. These aren't very squeezable and tend to dent from dropping or pressing. You can find PET bottles in every shape and size - Cosmo oval, Boston round, bullet (the picture to the right), and so on.
Good resistance to oils, but general more expensive. These are used a lot for shampoos and conditioners. They are squeezable, which makes them appealing for hair care products or other products when you want to get out every drop. Very good for small, sample sized bottles as well because you can squeeze it nicely. (And it uses less petroleum than the other bottles, so it may be considered slightly more environmentally friendly than the other bottles.)
These are much softer bottles and less chemically resistant than HDPE. They tend to be translucent, and are intended for squeezy applications.
Good for...when you need to get every drop out of a bottle.
Having said all of this, I still prefer using glass or aluminum bottles for something with lots of essential oils, but that's my preference.
Check out this great post at CPS Plasticology 101 for more, in-depth information on packaging!