Brennus is my choice for best leader in the game. He really is extraordinarily good, and an amazing headkicker. The trait combo is unparalleled, and they both work very well with each other.
Charismatic is imo the best direct warmongering trait, and the extra happiness is extremely significant in the early game too, in peace or wartime.
Spiritual is my pick for the game's best trait, especially for warmongers, with the ability to switch painlessly between peacetime and wartime civics for maximal empire efficiency. And easy religion switching to play the diplomatic game. Plus cheap temples never hurt.
They work well together in really giving you the most out of theocracy and vassalage, and the happiness helps with big cities or lots of whipping for slavery/caste system/pacifism switches, or quick bursts of drafting.
The Gallic Warrior is surprisingly solid. The "uses copper or iron" is very powerful in itself, not just because there may not be iron in your territory, but because if you have copper and make the effort to road it up, you can be churning out Gallics the very second you hit Iron Working (avoiding the massive lag time and resource cost of connecting iron or even settling a new city for it). If you've already been building a few axemen, a few Gallics will add a massive amount of punch to your axeman rush in that crucial period before defences have been significantly built up. And of course, because there's less attrition and because you're Charismatic and getting lots of promos, there's a lot more momentum behind the rush and you can breeze through more and tougher cities.
The "getting them early" aspect makes a remarkable amount of difference if you're in a position for an axe rush, and gives you the most powerful axe rush in the game.
As for the hill bonus, here's something I posted in another old thread:
The UB is no Sacrificial Altar, granted, but it essentially gives three more mini-UUs (archers, longbows, crossbows) which I think is pretty decent. I think it works well in combination with the UU, to create small forces comprising only Guerrilla II troops (and maybe a mounted unit). You've then got a little combined-arms strike force that can run swiftly over any hilly terrain to quickly get deep into enemy territory and do whatever. For all intents and purposes, a little group of GII troops (especially with archery units thrown in the mix) cannot be removed from a hill (except with serious losses). So hills become massively-fortified staging points deep in enemy territory, from which you can pillage and retreat, cut off enemy supply lines, harass and block enemy reinforcements, heal up your troops, steal workers if youre lucky (or just stop them working), and just generally be a complete and utter nuisance. You can also force them to build/keep more defensive troops in all their other cities, and seriously weaken their frontline defence (or even get lucky and take an inland city or two). It's the combo of the great defence and the fast movement that make it so very nice. And the terrain doesn't actually have to be particularly hilly for it to pay off either.
Remember that Brennus also will have some of the game's most highly-promoted troops, so your guys are likely to have other promos too - a medic to help with healing, some shock promos to take out passing troops, guerrilla III for removing other guys on hills, eg to clear a hilly path for your main force (GIII crossbows are niiiiiiice), a couple of city raiders just to be threatening, etc.
Guerrilla I is really just a nice bit of gravy for your main stack, and you probably won't want to go for GII for most of your gallics and crossbows (although GIII crossbows are excellent against hilly cities - and then frighteningly good defenders of the same). It's the versatility that really makes it shine. Oh yeah, and Guerrilla Gallics upgrade to Guerrilla Macemen too!