Tangela should not be S rank. I would probably move it down to A- or even B+.
I've tested out the Sunnybeamer set extensively; it wasn't consistently good. It has multiple flaws that prevent it from being worthy of S-rank. Being specially frail/weak to birds means that it either loses or is badly damaged by a lot of opposing late-game sweepers/random shit including Haunter, Huntail, Pelipper, Murkrow, Pidgeot, Plusle with HP Ice, etc. Sleep Powder is also unreliable; you have to land a 85% move, hope that the opponent doesn't wake up the turn that you Sunny Day, and hope that that check/counter that you slept was the only check/counter remaining on his or her team. Tangela also has way too many checks and counters to consistently weaken and/or remove to sweep; I've NEVER swept using this thing. Shit like Flareon, Torkoal, Haunter, Pelipper, Murkrow, etc/etc all check it and it's difficult to remove all of that. At least Haunter can spam Tbolt/Psychic on WishProtect Flareon to fish for para/spc down...
I haven't played with the Physically Defensive variant, though I assume it's also mediocre due to the lack of Synthesis and its frailty on the special side when compared to Bellossom.