First of all, I agree with Bonethor on the fact that synergy is everything. For example,
Foundation Breaker is amazing in Meren since I can repeatedly bring it back from the graveyard, and it triggers stuff that cares about creatures dying.
In general, if I'm building high-power casual deck or cEDH decks, efficiency is everything. I want cheap spells, even if maybe they can hit a more restricted number of targets:
Swords to Plowshares,
Deadly Rollick,
Chain of Vapor etc.
In casual EDH, I still value efficiency of course, but flexiblity becomes more important. I'm more than willing to pay a higher mana cost for a spell that can hit a higher range of targets:
Beast Within,
Generous Gift,
Chaos Warp etc.
This is my philosphy in general. So I would say that my priorities for casual EDH are
1) Synergy
2) Flexibility
3) Efficiency
And I usually don't like spells that hit multiple target. In particular, I'd never play
Decimate or
Hex since (and many people don't realize this) you can't actually even cast them if you don't have all the targets.
For example, you cannot cast
Decimate to destroy a land and an artifact: you also need a creature and an enchantment. This will mean that sometimes these spells will just end up stranded in your hand, which is terrible.
Also, usually you're not losing to multiple things, but to a single thing. Better to remove the one thing that's making you lose/stopping you from winning than paying more mana to blow up multiple things, making also more enemies at the table in the process.