That really depends on the champ that your playing. Yet some Keystones can be standard in certain positions. (I will speak mainly in the key stones.)
Top: Depending on whether you playing a Tank/Juggernaut or a poke/duelist. You would want either grasp of the Undying. (namely if your melee.) or Warlords Blood lust. (for ranged champs like Graves or quin.) Though if you take that occasional AP top laners, Thunder lords is a good choice.
Jg: Strength of ages, hands down. No other keystone really helps Junglers in the same way. Since just about every keystone is balanced around attacking an enemy champion.
MId: Depends on the mid laner. Thunderlords dominate Mid usually, but more DoT mages like Brand, Malz, Possibly even Cass, may do better with DFT. And AD midlaners may want to go Warlords.
ADC: Generally either Fervor of battle or Warlords. (offensive or defensive options.) But some caster focused ADC might take Thunderlords instead.
Support: Windspeakers, Thunderlords, or bond of stone. Depending if your going to be a sustain, poke, or a tank support.
In the end, you might need more than 7 to have all your basis covered, it depends on your champion pool.