C -> B-
Tentacruel can run two sets: Rapid Spin and Swords Dance (unfortunately it can't run both at the same time).
Tentacruel has a big advantage over Starmie, Cloyster, Forretress, Donphan and Golem as a spinner - it beats any set of both of the usable spikers. None of the others achieve this. This means that if you use Tentacruel as a spinner, Spikes will be out of the field if you want them to be. Period. Unfortunately, due to not being able to run Swords Dance and Rapid Spin at the same time (and not having any decent threatening options in its movepool), this means that everytime it comes in it you will be instantly forfeiting any momentum you may have - which it not exactly ideal in a spinner either. While not A material, it's still a great spinner that should always be considered, specially in stall teams.
The Swords Dance set can be very dangerous. This thing is very fast (speed ties Miltank and Zapdos), has Surf to make stuff like Steelix think twice before coming to phaze it and Sludge Bomb's poison chance is great too. Royally screwed by stuff like Raikou and Gengar and struggling to get past Skarmory (only 23% to 3HKO) are its main problems, but it can still be really dangerous with its speed and ability to boost its attack so quickly without becoming slower or sacrificing HP.
All in all a great Pokemon, definitely has its place in OU and should definitely rise in the rankings.
D -> C+
Another Pokemon that you don't see very often but performs its role well. Meganium has one purpose: support. Most commonly seen running a moveset in the likes of Synthesis, Reflect, Razor Leaf, Leech Seed, it does a great job at Reflecting in front of any physical attacker except Heracross, something Raikou struggles more with due to not having instant recovery and Earthquake being so common. Grass is also a great type defensively, resisting Ground and Electric moves (and although most electric type move users run coverage, it's usually weak coverage in the form of Hidden Power). The excellent screen support it provides is great for stall teams, who appreciate it to control a threat for some time, or sweepers, who may use the turns to set up a sweep to win. Underrated, fun Pokemon.
C+ -> B-
Jynx has one interesting niche over other sleepers - it screws up the most common sleep blocker, Zapdos. This means that Jynx is more likely to put something relevant to sleep. 110 base Special Attack is no joke either, with a great duo of STABs to work with too. It's also able to run Thief and steal an item from a counter - something like a Snorlax is a perfect Thief target - as well as Substitute to play mind games and shield itself from moves. Not the easiest Pokemon to use but it has its place in OU for sure.
Golem might deserve a rise to B- too - it's actually a good spinner, I just haven't used it enough to feel I can write about it. Charizard also seems better that the rest of C-, maybe it deserves C+? Not very sure on this one though.