Which style in the left hand is the most effective - a shield, a spell, or a second weapon?

In particular, I'm currently playing a heavy-armor mace-wielding orc. From the little I've tried of dual wielding, it doesn't seem particularly effective - the inability to block makes you seriously fragile... but it look so cool. Is it a build that becomes more effective as I invest in the dual-wielding perks? A shield fits nicely with the Orc Juggernaut archetype, but is kind of boring.

2 swords and you'll kill too fast.
Sword and shield you'll block more effective than just 1 weapon.
Sword and spell you'll be able to do more stuff.

This. I suggest the sword and spell (maybe switch it up from destruction though). It makes for a really rewarding play through.

Dual Wielding is made faster and more effective by Perks.

i typically duel wield with a spell hotkeyed for offhand. works for me. I'll get a feel of where I am and use a spell accordingly to the locale.

Dual-Wielding become stronger as you go and more effective against large groups of enemies.

Just go two-handed.

I'll either dual wield two swords or go spell and sword.

I'm having a lot of fun bashing things with the shield on my current character so I would go with that. In fact last night I killed a cave bear without a scratch by just eating a venison stew as he charged and then just bashing him every time he started an attacked. While he recoiled I would strike with my sword. Bash/swing, bash/swing. Loved it.

Spells are very fun to use coupled with a sword (or any other one-handed weapon). I suggest using them, but if you do, you should experiment with different types of magic (not just destruction). Playing around with different types of magic is the most rewarding way to play.

Yea, you want to give nightblade a try (illusion, destruction, alteration mixed with stealth) it's really fun.

I've never really played stealth before, but I should definitely give it a try.

If I'm using a sword in one hand I'll always have either a 2nd one in the other hand or a shield. Never tried the sword/spell combo.

I always favor going sword+shield, as I like having a shield to block enemy arrows/spells/power attacks with. I also like having a bit of a duel with my enemy, rather than going all out on offense until the enemy is dead.

Have never used a shield in Skyrim. Sword and spell.

Depends on the type of character.
Right now im dualwielding a sword and dagger but use some destruction for distance battles like dragons.

Everytime ive used a shield i feel overpowered for some reason, staggering is just way too effective haha.