While I don't have any real scientific evidence with graphical representations like SteveD above (great post BTW), I usually go between the vertically displayed labels versus the horizontal right-aligned labels (and left aligned fields).
My rule of thumb is if the labels are of similar size I tend to go with the right-aligned label/left-aligned field approach. This way readability is not compromised as much and it keeps the user flowing better through the form you want them to fill out. If the labels are of multiple sizes, then it's best to go with the vertical label approach. There are a few exceptions to this rule, mainly based on viewport size and content of the form.
Instance #1 -
When you are viewing the page on a mobile device, regardless of the original design, I usually switch to vertical labels. Since space is a big issue side to side, vertical labels tend to flow well on the smaller screen.
Instance #2 -
When you have varying lengths of labels, but these longer or shorter labels can be broken into different sections - I still use the right-aligned label/left-aligned field approach. For instance, you have a bunch of fields for contact information (e.g. first name, last name, address, phone number, email address). Then you have one or two fields asking for more detailed information (e.g. Special Shipping Instructions, Mother's Maiden Name, etc). These fields could be added below the first section separated by a horizontal rule. This way the fields are still on the same page using the same basic formatting, but the extra space and the horizontal rule help to make these fit together a little easier.
With all this in mind, to directly answer your question I would agree with SteveD above and say that Option 5 would be your best bet.
