Whatever the other guy doesn't have.

Or, to put it another way, there isn't one.

There is a lot of talk that says specialize in one troop, so you can focus on getting all boosts for that type. Seems sense.

There is a chart somewhere, pentagon shaped with arrows, showing what troop types work best against which. Probably in a vikings wiki.

e.g. Siege is 50% against Spear, and 25% against Ranged. Others have similar. When you look at a troop, this is the Effective and Weak against numbers shown at the bottom of the troop description.

Scout your opponent. Send what he is weakest against.

For the same troop type I expect it becomes his stats against yours.

Don't forget: If you can think of it, so can they. If you're loaded for bear, so are they. If you have remembered to apply Health / Defense / Offense boosts, so have they.

Make sure you're in the biggest order you can. Buddy up. If you're weak against your prospective opponent, speak so in order chat - someone with the better troop type will be around. They'll return the favour. You can even mix troop types via raid so your hero and stats combine with your order mate's.

My current working theory: Until you're big enough to have Legendary Amarian battle gear with full non-alpha jewels, including on your (Adrian) Inquisitor, your troop type doesn't much matter - everyone is bigger than you are, and multi-Billion players looking for somebody to steal from are on every street corner. So you're going to be shielded most of the time, and you're not going to have enough skills or gear or boosts to much matter. (So save them for building then studying in the mean time.) So, build up about 1.5M siege so you can carry away loot and yield - stick with level 6 camps, level 1 assailants (until you can kill one in one load). By the time you get anywhere near that, you will much better understand whatever suits you, going forwards.

Job 1: Legendary Savage gear - all gear comes from beating up on Assailants, and the Savage gear is the most effective to do so. And gives you a place to hang jewels off of in the mean time. [It is quite usual to have no other gear before this.]

GL&HF! :)

Best troops for offense depends on what troops you want to kill 

Siege so you can carry away the most loot and yielding. You'll be shielded most of the time, so what an opponent is using doesn't matter most of the time. You will know when this doesn't make the most sense for you for your playing style - and will be able to govern your by then better informed self accordingly. Or read the longer post above.