Ninetails: Low Tier
-Availability: Midgame. In Blue, Vulpix can either be found before Celadon City (where you can buy a Fire Stone and evolve him straight away), or in the Pokemon Mansion. In Yellow, Vulpix is obtained through the Game Corner, which is also in Celadon.
-Stats: Ninetails has very good Speed and Special. Its Special allows it to take Special Hits (provided they are not Water moves), and it outspeeds a lot of Pokemon. However, the rest of his stats are average at best.
-Movepool: If you evolve it into Ninetails straight away, you'll be stuck with Ember unless you feed it TM's such as Fire Blast, Body Slam, and Dig. Keep in mind the latter 2 TM are in very high demand in this game. Fire Blast also comes only later on in the game and has low PP, which may cause issues when fighting the Elite 4.
If you keep him as Vulpix or catch him later at Pokemon Mansion, you can train him to learn Confuse Ray and Flamethrower. However, Confuse Ray is not the most efficient strategy as it is a luck based moved. While Flamethrower may seem tempting, by the time you get it, Fire loses much of its value due to not being especially good against the majority of latter gym leaders/E4.
-Power: Not very powerful at all. Evolving it straight into Ninetails means you are stuck with Ember as your STAB.
-Type: Fire
Offensively, it hits Bug, Grass, and Ice for Super Effective damage. By the time you get Vulpix, you'll barely have any Bug and Ice types to fight. The majority of Bug users are battled before Celadon, where as the only Ice type you battle (that is weak to Fire) is Lorelei's Jynx. Grass is still common at this point, but you also get HM 02 Fly and TM13 Ice Beam to hit Grass types with. Fire is resisted by Rock, Water, Fire, and Dragon. Most Rock types you face in-game have weak Special and wont even like taking resisted hits, and there is only one Dragon user in the game. However, Water is problematic for Ninetails, especially with how common it is late game (all the water trainers in the water surrounding Cinnabar Island).
Fire also offers resists to Bug, Grass, and Fire. However, he is also weak to Water, Ground, and Rock moves, which is more common than Bug/Grass/Fire moves.
-Mathups:
--Erika: Despite the type advantage, Ninetails only has Ember which will not sweep her team. Also, status moves put a serious damper on Ninetails performance.
--Koga: If Ninetails was given the Dig TM, it can hit his entire team for Super Effective damage, just watch out for getting poisoned. In Yellow, Koga uses Venonats/Venomoth who are weak to Fire. However, much like Erika, you'll be stuck with Ember and Ninetails will not like being poisoned.
--Sabrina: Ninetails has the Special Bulk to survive Sabrina's move and can use Body Slam/Dig to hit her physically frail Pokemon.
--Blaine: Dig hits his whole team for Super Effective damage. While Growlithe and Ponyta are not threats, his Rapidash and Arcanine can survive a Dig. Rapidash and Arcanine also both have goo Physical Attack and can hurt Ninetails with Stomp/Take Down.
--Giovanni: Dig is Super Effective on his whole team except for Dugtrio. Despite that, Giovanni's Pokemon can take a Dig from Ninetails and wear it down with their own moves. His Dugtrio also knows Dig.
In Yellow, almost all his Pokemon know Earthquake, easily taking Ninetails out.
--Lorelei: The only one who Ninetails can safely take on is Jynx, due to her weakness to Fire.
--Bruno: His Pokemon have low Special and don't like taking Flamethower/Fire Blast. Might want to be vary on Fire Blast PP, though.
--Agatha: If you have Dig, it deals a lot of damage to her team as they are all weak to Ground except Golbat. Golbat isn't too hard to take out either, but watch out for Confuse Ray or Toxic.
--Lance: His entire team resists Fire and their powerful Hyper Beams will hurt Ninetails. Even worse, Lance's Gyarados knows Hydro Pump, which will make short work of Ninetails.
--Gary: STAB Fire moves can take out his Grass Pokemon, while Dig can hurt his Electric types, Rhydon, and Fire types (excluding Charizard). However, his Water types are a huge threat to Ninetails, and in Yellow his Sandslash can Earthquake you.
Additional Comments: Ninetails does well against some Gym Leaders/E4 members, but at the same time, its at the cost of using up two of the most valuable TM in the games - Body Slam and Dig. There will be plenty of other people who will want those two TM's, and can use it better than Ninetails. Hence why I'm placing Ninetails as Low Tier.