Alright, I'll bite. I'll only describe the builds where I feel the gun is a "perfect fit", where it fits like a glove and I wouldn't ever change it with another one.
Batarian Slasher: this guy can kill a gold Brute in less than 1 second. ... Do I really need to add anything more? This is definitely one of the contenders for the "most perfectest fit" title.
Human Soldier: yeah, he can use many guns effectively, but the Harrier... I have a hard time thinking of many other kits that are as good Harrier weilders as this guy. He's a pretty straightforward guy: just keep popping those heads and you're set. Adrenaline Rush damage bonus is extremely effective at reducing ammo consumption and at decreasing TTK at the same time. Even if you do the vast majority of the killing in your team, if you use your TCPs smartly and you aim for the heads you'll have zero ammo problems. And if you do, for some reason (looking at you, Condor/Hazard Dagger), then the Hurricane will be a good backup gun for that short while. Frags are very good for CC and to deal quickly with large numbers of mooks clustered together. CS is there for the odd moment when you need it, like when you need that Phantom to bubble RIGHT NAO and you have no frags left.
Turian Soldier: again, he can use so many guns so perfectly that it's hard to include a "perfect fit" build. I was about to leave him out, bu the Raider is just so goddamned good on him that I just couldn't. Strategy is, again, very straightforward: Marksman is your bread, butter, coffee, juice, cookies and pretty much anything else. MM is so great on this build that even when you're facing a boss, you're not really gaining much by using PM instead of it, unless there's someone else shooting it too. So you'll end up using PM once in a blue moon, and CS even less. But hey... The bright side is that you can OHK a Phantom across London, if your aim is true. MM turns your gun into a no scope penalty, pinpoint accurate, shieldgate-ignoring, hits-like-a-truck sniper rifle. It just... Feels so good. If you haven't already, try this out: you won't regret it.
BF3 Soldier: I'll say it plain and clean: this is not a build for the faint of heart. He's extremely squishy, and if you're used to using ARush as a defensive ability as well, you're gonna have a hard time. However... The prize is worth it. For this guy murders armor for breakfast. Pair him against reapers, and you'll be amazed by the speed at which he can whipe a whole late-waves spawn. Strategy is the good ol' double-tap claymore for mooks, with frags for clusters of them. Against big, armored targets like Brutes and Ravagers, you go Claymore -> AR -> Claymore -> hide reload in Carnage -> Claymore -> hide reload in frag grenade and... Pretty much anything in front of you is gone by now. I don't think even a Banshee could withstand the whole package. But you shouldn't be shooting Banshees anyway. Watch out for swarmers and goo. You'll really hate them after playing a round with this guy.
Destroyer: because a Destroyer without the Brophoon is like a Krogan who doesn't headbutts: it's just not right. Any time I see one, I die a little inside. The most convoluted tactic you'll want to employ with this guy is to use a TCP to hot reload the 'Phoon in the right occasions. As for the rest, it's just the usual "shoot the heads off". And use multi-frags on clusters of enemies. The Piranha is there for the odd time when you're out of bullets and you don't wanna waste a TCP.
Havoc: another contendant for the "most perfectest fit". Instead of trying to explain why with words, I guess I could just link this. The only downside is that it's very host-dependent, and sometimes can get funny even on host.
Geth Soldier: What? You're using a SR on a kit with Flamer? Are you nuts? Well... Yes I am, that's true, but this build kicks ass just the same. The combination of a SR and Flamer gives you very good and accurate damage output at range and hellish CC and damage output up close... Especially if you're good with those quickscope shots. A headshot from the Javelin kills any humanoid enemy in the game, and flamer takes out the bigger targets. You can hide the reload into flamer, and stop it just as you can fire again, so that your CD won't ever be too long. The flamer burst will still be long enough to do a lot of damage. You can swap the power amp for a cyclonic if you feel that you need more shields, or with an adrenaline module if you really wanna race for the fastest time possible. The downside is that it's a somewhat skill-intensive setup, as you're gonna be spending a lot of time firing your SR in CQC, which means you need to git gud with those quickscopes. If you do, though, an example of how this build works can be found here.
Quarian Female Engineer: Yeah, yeah, I'm playing the weapon and this girl is actually a mage with fire and ice spells. Still. This works like a wonder, but honestly I'm tired to argue with people saying it doesn't. Here is for you to judge.
Geth Engineer: another awesome fit. I could never get the hang of this guy, until I tried this loadout out. Talon on a kit with HM is just a completely different gun. It's lethal. Strategy is simple: overload to strip their shields, then pop their heads off. You can do so from quite a distance. Use the turret to get some shields back if you need it. Don't be afraid to pop one of your 9 TCPs to get that sweet 100% damage bonus in key situations.
Human Vanguard: a classic. Everyone and his mother have been using the Talon on him ever since forever, and with a good reason. Light, it packs a punch, it is plenty accurate enough for a in-your-face class like this, and, most importantly for this guy, it primes awesomely. Strategy is charge, shoot, nova, shoot, nova, shoot, rinse and repeat. If you're doing it right, you're gonna have a tech burst galore. Try not to shoot mooks in the face too much, as dead enemies can't be detonated
Also, do abuse Nova to cancel it and get free I-frames. If you do everything right, you're gonna be a train of destruction. Basically the only thing that's gonna stop you is getting bs from the game. And if you manage to get your hands on a pizza... Best pizza delivery guy in town.
Slayer: notice any similarity? Yeah, this guy is basically a Novaguard on steroid - without the I-frames, but with teleport. Everything I said about the Novaguard applies to him too. Just replace Nova with PD, and nova-cancel with the teleport dodge. Do abuse it. Also another handy feature is that his light melee, coupled with charge, will give you roughly 98% effective damage reduction for 4 seconds (after which you need to charge again if you wanna keep it up). It' more than enough to make you pretty much invulnerable unless you get staggered. Cerberus turrets will do 0 damage because of roundings, lol. Overall, not very efficient, but a useful "oh ****!" button.
Aaaaaaannnnddd... I think that's pretty much all? Any question, I'm here.