Online Curricula...

There is no "best" curriculum, even when it comes to online curriculum, because children are not all the same--what works well for one student may be a disaster for another. This list includes a variety of options that, based on our experience, are popular with homeschool families. (Being included on our list does not imply our endorsement of any.) Note that new options are popping up all the time but some are scams so vet them carefully before spending money on them. 

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These are listed in no particular order:

Time4Learning  A non-religious subscription online option that's popular with many.

Khan Academy A non-religious free online option (funded by Bill Gates and other wealthy and everyday donors) that some use as their main curriculum and others use as a tutoring option.

Miacademy An online subscription option. It has separate options with a program called Clever Dragons that is geared specifically for boys, another called Always Icecream for girls, and another called Mia Prep that's specifically for high school students.

ABeka Academy Online A popular Christian curriculum that's been around for decades.

IXL  An online nonreligious program known for its diagnostic testing to help determine skills and levels that the student needs work on.

Monarch Online A religious homeschool curriculum that has passed the test of time as it's been around a long time and has an online version.

Easy Peasy All-in-One Homeschool A religious curriculum available for free online. Workbooks can be purchased for those who don't want to do all online, but they aren't required. There's also a high school version: Easy Peasy All-in-One High School.

Oak Meadow Online A secular homeschool curriculum that has passed the test of time as it's been around for over 100 years and now has an online version.

Acellus Power Homeschool An online option that's been very popular with homeschoolers though it has recently changed so that it is less flexible and doesn't allow students to skip lessons they know well.

Education.com An online option for students in preK through 8th grade that gives access to a number of free resources and access to more come with a low cost subscription.

Study.com An online subscription option for high school students.

There are a number of options that do not cover all subject areas such as these for math:
CTC Math 
Mr. D Math
Math-U-See
Teaching Textbooks
Nicole the Math Lady
Math Seeds

For teaching reading and other beginning skills:
Reading Eggs
ABCya
ABC Mouse
StarFall

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For those wanting public school options:

CPalms A website with educational materials and links to Florida public school standards and test practice which can be used for occasional practice on a topic or as a curriculum but leaves the parent in total control of lessons done and deadlines and such.

Florida Virtual School (FLVS) A free (supported with Florida tax dollars) for Florida students (though students using state scholarships will have to pay since they are already getting tax dollars). 
FLVS Flex Home education students tend to use the FLVS Flex version which allows them to sign up any time of year for from 1 to 6 classes that the parent chooses and work on them at times and schedule that parent/student choose as long as the student keeps up with a pacing chart.

FLVS Fulltime, various county (aka school district) versions of FLVS, K12, and Connections Academy, are all fulltime public school online options. Those using them are legally considered public school students and must follow the public school requirements, take classes as assigned by the school on the days and times assigned, etc. They can be signed up for before a semester starts and not in the middle of a semester. These are not considered homeschooling since the parent does not direct the education even though it is done at home.