which of these tires by GRP are the best combo for grip and life: jolly medium compound, atomic medium compound, or cubic medium compound?

Jollys are a pretty durable tyre in the medium compound, the soft obviously works better though

i can't say for the new style Atomics & cubics as i've not tried them in the new compounds!
GRP have recently changed their compounds! the A is quite a bit softer than A bought last year!

which will last the longest?

the Jollys in the hardest compound you can get i suppose?

personally i run the tyres that work the best for the track and conditions that i'm running on that day!
sometimes that means a fresh set after every run, sometimes same tyres for qualifying & a new set for every final!

tyres aren't a buy once & run til they're bald item!

if you want your car to perform at its best you'll need to be prepared to buy a selection of tyres and you'll need to be prepared to change them regularly!

think of your tyre box like a tool box. ie you need the right tool (tyre) for the job in hand

jolly has the best life of the three, but the atomic will be a little more consistent as they wear (which is much faster than the jolly), and will provide better grip in most cases. altrough jolly has good grip in the right surface.
we use the cubic only when the track is muddy because they wear fast on dry, and provide very good grip on mud and the smal tread dont load up with mud as other tires.