Best civ for aggressiveness

[SS-18 ICBM]

I'd prefer getting March on my HAs, so they can be great Knights/Cavalry/Tanks later on. That paltry range attack is just costing you maintenance without killing AI units or even putting a dent on cities. Not to mention that trying to attack a fully fortified city two times (i.e. leaving your HA a sitting duck) will get the unit creamed.

 

[TallNSturdy]

I still have a soft spot for England, given the increased strengths of cities. Longbows early on can do wonders softening up cities before your melee units and catapults get in range. They keep the extra range when they upgrade to gatling guns, yes?

They keep the range bonus of +1 so they can shoot 2 tiles away unlike the longbow that can shoot 3. Still a really nice bonus, it's like really strong archers.

In a game I was playing as the Celts I had a CS gift 3 longbows to me early and I was able to keep them alive long enough to upgrade them to gatlings. Now that was a fun game.

 

[Kouvb593kdnuewnd]

As far as I know which is that their are no civ that are best at aggresion as pretty much everything else in the game it depends.

 

[The Pilgrim]

Huns and China get my vote. China was always a top tier military civ, but with buffed Archery line, IMO, it became number one really.
Huns... HA Mini-keshiks... (c) Enough said.
But you have to make sure you don't lose them. Even if at the beginning this means taking potshots at smaller targets and retreating to heal instead of capturing capitals. If you take left side of Honor and have barracks, logistics will be unlocked pretty fast. From then on it's a steamroll time. Rams are good for lower levels, on immortal/deity, they're eaten alive by spears and warriors. Not to mention horses...

 

[RealHuhn]

China!

Improved GG => Every unit is stronger
Cho-ko-nus (spelling?^^) => Level up twice as fast due to their double attack, unstoppable when upgraded to gatling guns with +1 range most likely
More GG => Citadel spam

Others were already mentioned. Russia is also great on higher difficulties just because they can sell horses and iron for huge amounts of cash and get the extra production, similar to Huns. One horse tile with +4 horses = 8 horses available for selling = 360 gold.
Keep a few iron for yourself and rushbuy a huge army of warriors upgraded to swordsmen. Add a few catapults and you should be fine.

 

[Erneiz Hyde]

My first choice is China. Double attack from the get-go is simply golden. You can spam archers in the early game and upgrade them all to chu-ko-nus very easily with the help of Paper Maker.

My second choice would be either Mongol or Arabs for their ranged knight. Mongols' UA isn't really useful, but they have +1 cavalry movement and the ability to move after attacking, which is pure evil against the AI. Not to mention Khans heal troops (if you have March in all your Keshiks and a few medic here and there you're simply unstoppable). On the other hand the Arabs have UA that can benefit in times of war (trough LOTS of puppets connected by trade routes) and powerful UB that benefit from peace (bazaars).

But thinking back about it, almost all (if not ALL) civ has a way in getting some edge in war, even those tailored for a more peaceful approach.

 

[KillingMeSoftly]

Anyone with two unique units are pretty much the best warmongers. This lets you leverage your advantage at least twice in a game or in interesting ways. Most people feel the ones with the earliest UUs are the best because this lets you stomp an neighbor or two really early and thus set up the snowball effect.

I like Germany quite a bit for all-out warmongering. -25% maintenance cost is very sweet and lets your economy support so many more units. Picking up free Brutes and upgrading them to Swordsmen is great as well, plus after the first ~20 turns barb camps start spawning with spearmen/archers. You can also beeline Civil Service to pump out a never-ended stream of Landsknecht. They are stronger than Swordsmen in G&K and if you get there early enough you will have a very potent army.

Some Civilizations that don't have two UUs are deceptively potent warmongers. China has already been mentioned many times already. The Incans are great as well because of ignoring movement penalties on hills makes for very interesting tactical choices, on top of them savin so much money on roads letting you field more soldiers. Along the same vein economically strong Civs can also make great warmongers due to having all that extra money. Making upgrading easier, flat out buying units, supporting more units, paying for other Civs to goto war, etc.