I've topped the ladder with my non-Tauros team before. He's good, extremely good, but he's not a cut above the other 3. Snorlax has a higher floor than Tauros (standard Snorlax will ALWAYS at least trade unless your opponent pulls miracle predictions) and is more versatile, Chansey walls 2-4 members of most teams and practically defines the meta. Egg... well, really, the issue with Egg is that either you're using a B-list Pokemon, you're using Egg, or you're playing 5-6. Hence, it's an automatic entry on most teams because there simply aren't any other good Pokemon with reliable sleep moves.
The "standard" RBY team is Tauros, Chansey, Snorlax, Exeggutor, and 2 others of choice (the canonical version having Golem/Rhydon in one slot and Alakazam/Starmie in the other). So those four do belong together in S rank.
Raichu's a solid D rank, yeah. I'd certainly use Raichu before I'd use Venusaur. It's crap, but it's unique crap.