A top five is really hard. It depends so much on what you want the mana to do.
The Elementals are all basically offensive, Water has an economic perk and really nasty summons if you get the Tower of Elements, its probably the best TOM choice. Air does area effect damage, Fire does collateral, and Earth does high STR damage and is good for mage XP.
Death and Sun are the best offensive magics if you have lots of nodes, Sun creates the highest STR units and death gets and affinity+ in its tier II summon. So, quantity or quality. Death has liches which makes it hands down better, but it has a diplo penalty. Death is more available though with the ToN and the Soul Forge. Death and Sun have less versatility than the elementals though.
MetaMagic is the ultimate attack magic for the non attacker. Djinn get universal affinity so you still get a powerful archmage summon if you stack Law\Enchantment\Life\Mind\Spirit economic manas. They are weak out of the box though so you still need a lot of mana to make metamagic good.
Entropy and Law are both good to have for their Mage summons, Entropy makes better mages with rust and wither but law has a better perk, -5% maintainence can add up in a large empire. 4 Law mana and Order basilicas means no maintainence anywhere, which is pretty darn insane.
Life is great anti undead of course, but it also makes for a really powerful AV civ since you can sanctify away hell and still sacrifice the weak. Unless you totally wipe out hell this is micromanagement heavy though. Still, for evil with hell in the world life1 is the ultimate economic spell.
Spirit lets you go for the dragons horde really early, plus it perks your healing, Spirit1+Body2 make for really fast regeneration. Spirit is a nice line all around, though I can't say +5% GPP ever really adds up and since its T3 spell is single use you wouldn't want to spam spirit anyway. Body is hands down the best army assist and interesting T3 offense. Be sure to cast haste right after you step onto that forested hill and have no movement points left. Before I knew about the dwarven repair spell I relied on body mana to heal my golems
+1 Happy is pretty good on its own for Enchantment and it perks two unit lines. If you have a lot of it for Happy sake you can get spellstaves too, which doubles your initial offensive to crack big stacks.
Mind looks good but I can't say much from experince, its probably the only sphere I haven't really used. +3% research is a minuscule perk, I wish it was at lest +5% so you could stack it to some effect.
Chaos I is really good, and II can be great, III is a little dangerous but it fits Chaos well. Its a good sphere.
Nature is popular, but I can't say I'd go out of my way to get it. T1 is situational, T2 is very good but not enough to make the sphere shine, and T3 can be gotten via Genisis with a lot less micro and no wasted archmage promotions (Unless you just have to have grasslands everywhere, but I think plains are good tiles so I don't bother with it)
Shadow is pretty bad, blur is very good, Shadowwalk doesn't do nearly as much as it seem too. Mistforms are weaker overall than most T2 summons so they are pretty much good for picking off workers.