I think Leon's just boring but practical. He doesn't get to make crazy shit happen like Claire, but his kit is solid, and somewhat easier to use. The Matilda carries a lot of ammo, which WOULD save you space, if you don't feel compelled to slap the Stock on. But if you're gonna use it, you probably should slap the stock on. Stock Matilda can put out some alright damage and stagger on whatever you need quickly. It's not amazing, but it'll get the job done against dogs and zombies you intend to run by.
The shotgun also carries a lot of ammo with the stock. Great for headpops, and it basically lets Leon clear rooms at will. It sits in a weird spot against bosses where it just looks like the Matilda can put out more DPS, but I guess you could use it in a pinch. It won't stun them often, though, so don't gamble your survival on that. It also won't stun Lickers, I vaguely recall, though I might be wrong on that one.
The flamethrower is really weird. It's DPS is absolutely godlike, but it has three horribly annoying niggles: It'll block the fuck out of your field of view, it'll make hard to tell what enemies are gonna do when they're engulfed in flames, and it seems to not stagger at all, ever, anyone. Except the Ivies. The Ivies burn good. You still wanna use it against bosses, though.
The M19 is really, really good if you go for Leon B. Not just because it changes Leon's run cycle to something less stilted and he aims it gangsta style, but because it puts out really respectable damage for an ammo that's pretty common. I still go for kneecaps over headpops with this, but if you absolutely need a zombie removed and don't have the shotgun yet, this is not the worst possible choice.
The Lightning Hawk is a good magnum. Free headpops on zombies with a headshot from pretty much any range. That would be wasteful, though. Excellent damage. Save it as much as possible for bosses. There are two really annoying Lickers at the NEST facility, near where the trophy sits, I tend to use the Magnum to clear them. Gunpowder yields kinda suck, you'll get 3 bullets if you're lucky.
Ada's part is murderously difficult as it throws you into a multi enemy gauntlet with only ten bullets. It's easy-ish when you know how to do it, but you'll often need retries.