This guy was brought up a few pages ago, but I guess I'll chime in with my experience.
Binacle: B/C
Tier
Availability: He's available on Coastal Route 8 or Ambrette Town by Rock Smash, so a little bit before the second gym.
Stats: 72/105/115/54/86/68 as Barbaracle, so a good physical attacker (boosted by Tough Claws), with solid physical bulk. Speed is a little low, but it can be patched up.
Typing: Water/Rock is pretty decent offensively
Movepool: It's level up movepool is interesting, it starts with Slash, which is nice when boosted by Tough Claws, and gets a water STAB in Clamp quickly after that turns out to be quite nice. Aside from the nifty Hone Claws at level 32, it then gets pretty dull until level 44, when it starts a sequence of Night Slash, Razor Shell, Cross Chop, and Stone Edge, all of which have their uses. It can also gets the amazing Shell Smash if you have a Heart Scale to spare. As for TMs, you get Rock Tomb quickly after, which is a serviceable STAB for most of the game, as well as a plethora of coverage moves (notable ones are X-Scissor, Poison Jab, Earthquake, Aerial Ace, Brick Break, Shadow Claw, Power Up Punch), many of which are boosted by Tough Claws and can be used to fill in whatever hole he needs to fill.
Major Battles: Grant: Clamp and Rock Smash make him usable
Korinna: No
Ramos: Preferably no, although I ended up beating his weakened Gogoat with this guys X-Scissor.
Clemont: Doubtful
Valerie: If you have Poison Jab, he wrecks this gym.
Olympia: Same as above, but use Night Slash/X-Scissor
Wulfric: Beats everything but Avalugg reliably
Flare: Depends on what coverage you have at the time, but should probably stay away from Scraggy, Croagunk, and Electrike. He can definitely beat the Houndour and Zubat easily. Lysandre is a sweep minus Mienshao.
Rivals: Not really any bad matchups minus Chesnaught. Can do pretty well if he has X-Scissor for Meowstic and Absol though. Again, can use TMs to your advantage.
Malva: I tried using one member of my team against each member, so she's the only one I have experience with, but obviously he was excellent here.
Diantha: Poison Jab saved me against her Gardevoir, can potentially beat Aurorus and Tyrantrum as well.
Additional Comments: Out of Greninja/Diggersby/Pangoro/Barbaracle (I know there's major flaws, but I went with the most interesting to me first run), this guy was my favorite to use from Valerie on, and I completely forgot to give him Shell Smash. There was a definite awkward point for me the few levels before he evolved, with all the fighting, grass, and electric-types making it hard to get solid exp. However, once he evolved, I basically kept a designated TM slot and plowed through the late game, even without Shell Smash. Not to mention that physical bulk came in handy for me quite often. There might be a bit of recency bias in B-Tier, which is why I think C-Tier is an option, but his effectiveness lategame was impressive.
Oh, and this guy has some merit as an HM slave.
Psyduck: C
/D Tier
Availability: He's available on Route 7 next to the daycare in a Horde.
Movepool: He learns Surf, Strength, and Waterfall, as well as useful TMs like Rock Smash, Flash, and Dig. With Hawlucha being beast with the HMs, Psyduck fills in all the other holes, with Rock Smash for earlier stuff, Flash for pseudo repel, and water navigation.
Additional Comments: Didn't battle with him so I'm ignoring everything except movepool. This guys main appeal is "synergy" with the other HM slave in Hawlucha. The only things Hawlucha doesn't get are the Water HMs and Flash, and Psyduck is the earliest thing that gets these three. I should note he doesn't need to evolve to be useful, so you can easily catch him and ignore him. He doesn't get Cut, which is unfortunate, and I don't have the battle experience to judge on C-Tier, but he is useful if you choose Hawlucha (or Farfetch'd I guess) as your other slave.