Guardian is really cool. I think it looks better in D1 than in D2. It's more photo realistic. This spell isn't very effective though unless you get it as highly upgraded as possible. But having a shitload of them firing all over the place can totally mop up some monsters.
I like Elemental, even though it was practically useless. It simply looked really cool. Golem was a fun spell, even though the programming of those guys made them pretty frustrating to control.
Detect Traps was by far the best spell in all the game. Just kidding. That spell sucked ass.
Aye even is the catacombs there are things heavily resistant to lightning. But for things that aren't immune to it it's great.
Agree about guardian. Helps for shooting things round corners too!
And detect traps was a passive rogue skill, dissarm traps was the actual rogue skill. being able to detact traps is pretty handy really. before I got mana shield my mage died twice to nova traps >.<