-Tauros - Mid/Low(?) Tier
-Availability: Tauros is available rather late in Fuchsia's Safari Zone, and is difficult to catch, having a low encounter rate and an equally low catch rate, which is quite a problem for Safari Zone. It's also slightly underlevelled when it's first available - a problem that can be fixed by having Tauros battle the Silph Co rockets for example - but Tauros's availability is a serious impediment to the player as, unless one gets lucky, Tauros costs more time than it could reasonably safe with its contributions. For what it's worth, Safari Zone is a location visiting which is compulsory for progressing the storyline, but blessed luck is still a must to catch Tauros in a timely manner.
-Stats: Tauros is a monster offensively and defensively, capable of outspeeding a lot of the fastest Pokemon with a slight level lead, hitting hard with a high crit ratio of 21,5% and taking physical hits nicely with its high defence. Its special stat is only average on the other hand (the only gen where it has a good special attack), but it can count on its sheer offensive power to combat specialists before they are of any danger.
-Movepool: Natural learnset is a problem for many Pokemon in the first generation and Tauros is no exception, probably packing only Tackle and Stomp when it first joins, and then learns Tail Whip, Leer (talk about redundant), Rage, Take Down. STAB Stomp should be more than enough to deal with most trainers you face, but you will have to give Tauros highly competed TMs such as Body Slam, Double-Edge and Earthquake. RBY is also the first and only generation where Tauros can deal decent damage with Fire Blast, Ice Beam, Blizzard or Thunderbolt, but there are better candidates for these moves. Doesn't get Surf in this gen, so it can't take a powerful special move for free, which is a shame. Also doesn't get Dig, surprisingly.
-Power: Assuming a good investment in experience and TMs, Tauros boasts highly competent attack and speed letting it sweep through enemies without necessarily hitting effectively.
-Type: Having a normal-type STAB and actually good offensive stats to go with it is invaluable, with Magnemites still being purely electric-type. Being normal-type also means being weak only to fighting-type moves, which are underrepresented in RBY. Needs Earthquake to hit ghosts, though. Doesn't resist anything besides Lick.
-Match-ups:
Erika (assuming you skipped her earlier) - Victreebel's Razor Leaf and Vileplume's Petal Dance hit hard and both are capable of sleeping and paralysing you. Given a high enough level, Tauros can OHKO with Body Slam, or it could get lucky with a crit, but it's nevertheless not the most reliable Pokemon for the task. RB Tangela runs only Bind and Constrict, so all she can do is waste your time a bit.
Koga - Assuming you visited Silph Co first, Earthquake can take care of the Pokemon here. Tauros should take a Sludge or two just fine if it doesn't OHKO. Koffing family's high defence don't make Tauros an attractive choice without EQ however. In Yellow, Venonat and Venomoth aren't any threat at all; you could even Stomp them or something.
Blaine - Fire Spin can be annoying if you don't OHKO. Earthquake is highly useful here, and in a match-up like Tauros vs. Arcanine, the levels and whether somebody gets a crit decide the winner.
Sabrina - Tauros is a worthwhile investment as the opposition here consists of glass-cannons, so a high-levelled Tauros with Body Slam and/or Double-Edge can OHKO the frail specialists that otherwise hit quite hard. In RB, the strongest move Alakazam and Venomoth have is Psybeam though, Mr Mime having Confusion, leaving the Kadabra lead as the sole proud Psychic wielder. Hopefully you go first and OHKO him with Stomp.
Giovanni - In RB, Giovanni forgot to give any decent STAB to all of his Pokemon besides Dugtrio. With Earthquake or Ice Beam/Blizzard, Tauros is a good choice for this gym. It's immune to Fissure/Horn Drill thanks to its high speed. In Yellow, nearly everything is given a good STAB move and the Nidofamily comes with Thunder, so Tauros is a little less safe if it fails to secure OHKOs.
Lorelei - the water-types here have good bulk and the movesets are vastly different depending on the version. RB Slowbro can only attack with Water Gun, while the Yellow one runs Psychic and Surf in addition to Withdraw and Amnesia. Thunderbolt, if you taught it to Tauros, won't be too destructive unless you crit, so if you wanted to sweep the whole thing with Tauros, you'd probably take an X Attack when up against the trivial Dewgong lead, and sweep with Body Slam/Double-Edge from there. It's best if you leave it to your electric though and just let Tauros take out Jynx.
Bruno - Watch out for Hi Jump Kick and Machamp's Submission in both versions. Hitmonlee is too frail and dies easily should you take him on with Tauros. The rest pose no offensive threat whatsoever, but you should abstain from facing Onix unless you have Ice Beam/Blizzard.
Agatha - the Gengars are very annoying. Hopefully you outspeed and Earthquake them all. Use your Poke Flute if you get slept. Arbok and Golbat shouldn't be any problem to you.
Lance - if you did teach Tauros Ice Beam/Blizzard or Thunderbolt, there are two 4x weaknesses to these here that you could exploit. Tauros takes Hyper Beams well, and could use its STAB for hitting everybody besides Aerodactyl neutrally if you didn't teach it any special moves.
Rival - He doesn't kid around this time, with his starter, Pidgeot and Alakazam now running their strongest STAB. Pidgeot shouldn't be too much of a threat and hopefully you can outspeed Alakazam and KO it first. Rhydon, Arcanine and Exeggutor are a joke however, with laughable movesets, and it's safe to switch Tauros in against any of those. If playing Yellow, better avoid Sandslash and Vaporeon (good bulk and can really hit back) and focus on the fire/electric mons.
Additional comments: Tauros takes a lot of time (real, not on cartridge) to catch and needs TMs to work, but if you're lucky with his capture and make the appropriate investment, he can really carry your team. Top tier combat impeded noticeably by hurting the efficiency of your run.