Let's look at the strengths and weaknesses of each type of power:
+ Fast to spin up
+ Can produce exactly the amount of power needed
- Makes Captain Planet cry
+ Produces a lot of energy
- Doesn't throttle well
- Pollution problems solvable, but not currently solved
+ Relatively reliable output of power
+ Hydro dams can be used as storage (pump water back up when it's cheap, let it drop again when needed)
- Reliant on geography
- Hydro dams are expensive
- Hurts fish
+ Reliable power output
+ Usually no pollution
- Only works in some areas
+ The greenest
- Not reliable
- Tends to produce most when price is cheapest
+ Clean, reliable and powerful
- Doesn't exist yet
From this, you can easily see that you can only get a homogenous power grid that works by using fossil fuels or massive storage. Everything else has problems either with reliability (solar/wind/tidal), throttling (nuclear), or geography (hydro/tidal/geothermal). Since storing electricity is hard, it is obvious that some sort of hybrid system is necessary.
The base load should be handled by geothermal, tidal, hydro, and nuclear, in that order (which is both how green they are, and how strict they are in where they have to be located). These produce reliable amounts of power. Tidal is a bit weird in that it's cyclical and not continuous, but that's not as much buffering needed, and it doesn't synch up with solar/wind.
Peak load should be handled by solar and wind, buffered by a reasonable amount of storage. Long-distance superconducting lines might also mitigate the problems - if it's cloudy in Paris, see if it's sunny in Tripoli.
And yes, there's still a place for fossil fuels. They're your last-line-of-defense system. Keep a few plants idling, to handle any peak load that spills over from solar/wind. Keep another few mothballed, able to be brought up within, say, a week. That way if your wind production gets wrecked by a hurricane, you can get back on your feet quickly.