Finally, something I'm qualified to comment on...
Seconding this. Got Magnemite before taking on Roxie for the sole reason that it could take on those Poison-types with ease, ended up keeping it for the whole game. Sonicboom is brilliant at that stage, since it does a fixed amount of damage it can 2-3hko anything reliably til you get past Burgh. Sure, it's totally useless against Clay but it's not like you have to solo the game with this guy.
Volt Switch is a welcome addition to its movepool after beating Elesa since it's more powerful than Thundershock is at that point and allows you to bring in a teammate to gain some more experience or turn a bad matchup in your favour (although once the game is in English or you have a translation patch this problem is mitigated somewhat). Thunder Wave is great too, I didn't make TM13's mistake and kept it til the endgame and it helped immensely.
Thanks to Chargestone Cave you can have a Magnezone right before the gym this guy has the easiest time against and blast through it right away. He resists Drayden too but I think his Druddigon's got flamethrower so watch out there. Marlon's piss-easy too.
Sturdy is ridiculously useful, sometimes surviving that one hit and getting off an attack or paralyzing your opponent will be crucial to winning. Sure, Steel/Electric coverage isn't the best but you can manage most of the game with Mirror Shot/Flash Cannon and Volt Switch.
Definitely
Top Tier.
Now to review a couple of other guys I tried out:
Snivy
Gave this fella a go because I've never used the Snivy line in-game before. Dude is weak as hell and it sure shows. You practically need Coil to be effective. Being weak to two early Gyms really doesn't help his case either. Elesa's gym would be better if Zebstrika and Emolga didn't have moves that can hit super-effectively. Clay is manageable, although Excadrill's still neutral to your STABs so watch out there. Don't even go there with Skyla...
For most of the game I was running Coil/Leech Seed/Leaf Blade/Return (never did find the Aerial Ace TM) and it sort of worked. Once you get off a couple of coils you can dish out some damage and tank physical stuff, but you're pretty much relying on being able to set those up or the damage output is low.
A barren movepool, only one STAB and some quite bad weaknesses (Fire, Flying, Ice, Bug, Poison) sound bad, but being fast definitely helps Snivy's case. Outspeeding stuff usually means you can get those Coils off, especially against the random Trainers who have no idea what they're doing. Then you can start tearing shit up. Til then, you're stuck with a relatively weak starter.
I'd be inclined to say
Low-Mid since he does have a few perks, namely Leech Seed recovery, 113 speed and some bulk.
Riolu
Got this bad boy as the first thing I ran into at Sangi Ranch, like Magnemite he became a mainstay. Sure, he doesn't get a STAB til it's too late for Cheren's gym but once you get Force Palm things get a little easier. Happiness seems to be a lot easier to increase now too, I had a Lucario by Level 20 and it rocked.
The one bad thing about Lucario is that he doesn't really match up well against any of the Gym Leaders at all except for Cheren. Roxie and Burgh resist him, Elesa's a bitch, Clay can take him out with Bulldoze, Skyla also resists his main STAB, Drayden's got that Flygon and his Druddigon has Revenge and finally Marlon's Jellicent hates Luke too. Yeah, not a great look. He does do well against the likes of Zinzolin, Corless and Grimsley though.
Lucario's movepool is damn good, once you beat Clay you can have Bulldoze in there so you can actually beat Poison-types properly, there's a bunch of Move Tutors if you're willing to grind for Shards (Ice Punch and ThunderPunch can be gotten as early as Driftveil) and there's Special moves galore too. You can stick with a Swords Dance Luke like I did (you only have to wait until Level 37 for that, which is around Mistralton normally) or you can try for Calm Mind (but for that you'll have to wait til Level 47, I would say Nasty Plot as well but that requires sticking with Riolu til the late 40's).
Gotta be careful with Lucario though, he's the ultimate glass cannon, one hit and you'll probably be KO'd. Drain Punch can help (although that comes way late in Seihaga City and requires quite a few Shards) but usually you'll wait to try and set up against something you know can't touch Luke and then proceed to plough on through.
I'd say
High-Mid taking into account the amount of Gym Leaders that he can't beat himself and overall frailty. He blasts through those ordinary Trainers though.
Crustle (Yes, I'm being incredibly specific here for a reason)
The Crustle you can catch in the Seaside Cave is no use against Marlon for obvious reasons, but just you watch as he ploughs through Team Plasma's Frigate all on his own (this is precisely what I ended up doing). Just one level off Shell Smash when you capture it (easily remedied with a Rare Candy) and equipped with Sturdy, you can almost guarantee a sweep. Hell, sometimes you can manage two Shell Smashes and almost everything will fall to a Rock Slide, X-Scissor or Bulldoze. Caitlin got absolutely crushed by Crustle. Oh, and so did Black Kyurem. Couldn't KO me with its Fusion Bolt, got off Shell Smash, managed survive another hit and then BAM, dead Kyurem. Didn't go nearly so well vs Ghetsis, mind.
One thing that sucks about Crustle is his speed. 45 speed is slow. With one Shell Smash you bring that up to 90, which can put you past most of the weak stuff the Plasma Grunts bring out (and the majority of Caitlin's team) but against faster opponents you could well be setting yourself up for nothing but disappointment...
Still, I'd say
High Tier simply based on how destructive Crustle can be if used properly.
Mandibuzz (again, that one specific one)
Here's a version-specific one, and one that you poor saps with White 2 are missing out on. I may be biased since Mandibuzz is one of my favourite Pokemon of all time, but I was willing to bend time for this chick and things worked out perfectly.
Mandibuzz has some seriously amazing bulk, it was taking super-effective hits and not even being 2HKO'd! Initially I used it as a Fly slave but when it got Nasty Plot, Dark Pulse and Air Slash I couldn't turn that down, especially not with the bulk to get off at least one guaranteed, if not two. Even though the special side is Mandibuzz's weaker attack-wise, after a couple of boosts not a lot can stop it.
Matchups against Gyms aren't usually kind, Elesa outright wins, Clay's stuff all has Rock Slide, there's no way Mandibuzz can beat Skarm one-on-one, Drayden is generally evil and Marlon I suppose is neutral. At least you can take down 3 of the Elite 4 with relative ease (and a bit of help from Weak Armor in my case).
Weak Armor may seem like the worst ability for it considering the defensive role it plays but it actually really helped. Sacrificing a bit of defense for Speed, especially after its special attack has been powered up, helped me power through both Marshal and Shauntal (I got lucky with the latter due to Drifblim deciding to use Acrobatics over any of its other moves...).
Leaning towards
Top Tier here, walls don't usually do too well in-game but this one sure as hell does. Just make sure you're around Route 4 on Thursday or willing to change the clock.
Vanillite line
I got lucky running around outside the Giant Chasm and found a Vanilluxe in the shaking grass, so I caught it and decided I may as well give it a try seeing as the Champion has a ton of Dragon-types. Unfortunately this thing comes WAY too late to be of much use...
You can train Vanillish/Luxe up quite easily in Victory Road (Golurk, Tranquill and Altaria are your friends) but apart from that, there's not much Vanilluxe actually does. Theoretically it would do well against Iris but most of her stuff outspeeds you (and Hydreigon is IMMUNE TO MIRROR COAT D:). Having a moveset that's made up solely of Mirror Shot/Flash Cannon (Which I didn't get the TM of), Ice Beam/Blizzard, Hail, Mirror Coat and nothing else of note just isn't gonna cut it. Hidden Power in-game is garbage of course because you don't know what you're gonna get unless you're RNG'ing, which really defeats the purpose of 'in-game'.
Low Tier because it comes so late and offers so little.
Among other things I had Magby/Magmar, Onix and Pellipper in temporary spots but didn't use them enough to really offer a critique, the former two because they require trades to reach their final forms and the latter because it was purely being used as a Surf-Fly mule.