I assume we're talking graphics first and tactical value second
I'd have to go with implosion and meteor shower. Earthquake also stood out. Given how it appears graphically though, you'd expect it to damage defenders (sigh).
I was kinda underwhelmed by icebolt and circle of winter. The ice seems the wrong color (clear/white is probably realistic, but hardly exciting), and the effect itself is kinda dull imho. Implosion, on the other hand, looks incredible. Ditto with meteor shower. Both of these are more like what you'd seen in, say, a mid era final fantasy
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. I found lightning satisfactory, though I can't recall what chain looks like off the top of my head. Haven't had access to it often. And I've been afraid to use armageddon when I had it (is there ever a good use for that?). Perhaps now that I'm in the dungeon campaign I'll find the damn spell and have dragons at some point.
Graphically, I found that too many of the dark buffs look very similar. When an enemy uses one, I still can't figure out which it is without looking at the text. I've been disappointed with a lot of the light spells too. Haste looks kinda wimpy and Righteous Might needs some more "ooomph" badly.
40 spells seems slightly modest, but only slightly so. Given the mechanics of the game, there's only so many things you can do on the battlefield (affect stats, do damage, etc), so more spells might start to duplicate things that are already there. I'd love to see more strategic-level spells though. Master of magic (which, granted is almost a different genre of game) had all sorts of spells that could be cast out of battle. Long-term buffs, spells that affected movement, spells that aided a specific alignment, etc. There's room for a lot of spells of that type, though HOMM has traditionally been light there, and if one thing is clear, it's that the brave developers of HOMM5 are ever so slightly loath to introduce drastically new concepts into gameplay
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