[RedMeowth]

I nominate Tentacool for Mid Tier, at least.

Availability: You can catch a Tentacool as soon as you get the Old Rod from Dewford Town (just before your second badge). He’s also extremely common to find in areas that you have to Surf on, which would be after the 5th badge and throughout the last 30% or so of the game. I should note that by THIS point, Tentacool is actually more useful than when you can first catch him.

Stats: Great speed and fantastic Special Defense. 80 Special Attack isn’t great but it’s good enough for him to launch Special attacks.

Movepool: Here’s where Tentacool starts to flounder. If you catch Tentacool as soon as you can, he will only have Poison Sting and maybe Supersonic. His level-up movepool is pretty barren and pointless up until Level 25, when he FINALLY learns a decent Water move in Bubblebeam. Until then, he has to make due with Acid and Cut for damage. The rest of his level-up pool is made mainly of support moves like Confuse Ray and Barrier. Where Tentacool / Tentacruel shines in moves is the TM moves he can learn. Ice Beam, Giga Drain, and Sludge Bomb are all good choices for the jellyfish, although the latter two are scarce. If you’d rather not use Tentacool for battle, though, he also makes a very efficient HM slave, learning Cut, Surf, Waterfall, and Dive. Between him and Tropius, you can have every HM move you could want in just two Pokemon (it’s a bit late for Flash, but nobody cares about Flash).

Type: Tentacool is a Water and Poison Pokemon. It’s nice early on to have at least one Pokemon that can’t be poisoned, and Water Pokemon are themselves resistant to Water attacks, which will be very common late-game, especially in Sapphire and Emerald. On offense, it’s actually a pretty good combination, too. Poison attacks will basically hit everything that resists Water aside from other Tentacool.

Match-Ups:
Brawly: Poison Pokemon don’t have to worry about taking much damage from Fighting attacks unless Tentacool is totally untrained.
Wattson: Although he has high Special Defense, Tentacool will not appreciate being hit by Shock Waves or Sparks. This is not his fight.
Flannery: If he knows Bubblebeam, this is actually a great chance for him to gain experience. Torkoal’s Body Slam can be a pain if she uses that, but being tolerant to Overheat is a big help here.
Norman: Not a stellar match-up here, due to Tentacruel’s rather low Defense. Slaking and even Vigoroth are dangerous for him.
Winona: At this point, Tentacruel should know Surf. That might afford him a chance against Winona’s Skarmory. If he knows Sludge Bomb, it will be useful against Pelipper and Tropius (in Emerald). Beware Altaria’s Earthquake, though.
Team Magma: The Numel can use Magnitude or Earthquake. But they are extremely slow and should die to a single Surf. Pretty even match-ups with Poochyena and Zubat that tip in Tentacruel’s favor.
Team Aqua: The Carvanha and Sharpedo resist Water moves, but they are rather weak to Sludge Bomb, as well as other non-contact physical moves.
Tate & Liza: On one hand, Tentacruel is weak to Psychic moves. On the other hand, he has high Special Defense, and he can use Surf to deal a lot of damage to Lunatone and Solrock. In Emerald, he won’t want to be on the front lines, but once Claydol (and maybe Xatu) goes down, he can lash out against the meteorites.
Juan: Resists Water Pulse and strikes back well against most of the rest of his team. He should avoid the Ground-type Whiscash, however.
Sidney: Tentacruel is still weak to physical attacks, which Sidney’s Pokemon typically favor. Surf for Mightyena and Absol, Sludge Bomb for Shiftry, Cacturne, and Sharpedo / Crawdaunt.
Phoebe: Tentacruel’s attacks aren’t really getting past the Dusclops, and one of them knows Earthquake. Fairly even match otherwise, but it’s more in Phoebe’s favor.
Glacia: Tentacruel takes Ice attacks (other than Sheer Cold) like a gangster of the sea. Sludge Bomb is probably not going to be good enough to fight the Sealeo, though.
Drake: Unless Tentacruel knows Ice Beam, this is a battle in Drake’s favor. Most of his Pokemon are physically based, and all of them resist Surf except Flygon, who also strikes one of Tentacruel’s weaknesses.
Wallace: Once again, he tolerates Water attacks well, especially coming from the weakest Champion ever. Whiscash is dangerous, as usual. Very nice to have super-effective attacks for Ludicolo in the form of Sludge Bomb.
Steven: Half of Steven’s Pokemon are weak to Surf, and the other half are hit for neutral damage. The closest thing he has to a Water resistance is Cradily, and she’s still hit for neutral damage from Surf or Sludge Bomb.

Overall: Tentacruel is not a massive juggernaut that plows through everything. In fact, he’s quite squishy at first. But once he's trained up, he is tough against powerful attack types like Fire, Water, and Ice and deals surprisingly impressive damage right back. He also finds value as an HM slave if you would rather not fight with him, but Tentacruel is a useful Pokemon no matter which way you use him. It is hard to ignore his patheticness in the first 25 levels of life, though.