Edit : Anyone saying Rhino is bad ain't a true player. Rhino is endgame mvp by very, very far.
I think it was less saying Rhino is bad and more that he teaches bad habits. Unless your future armory is just going to be Nezha, Rhino and Inaros, you probably don't want to get too used to being invincible and CC immune early on. I also think Rhino is so powerful during the starchart that he ruins what little challenge the game has to offer, but that's just my personal take. Other than that he's definitely really good, but I think most people are better off with Inaros once you start hitting sorties. No setup, no enemies or mission modifiers that can shut you down, and a better stun. Rhino is incredible in Arbitration, though.
Anyway, here's what I've found to be "the best" for common activities:
-Inaros or Rhino for anything with challenging damage.
-Rhino for Index.
-Ivara for Spy.
-Nezha for anything that can be sped up by running faster (especially Capture and Rescue).
-Titania any damage-gated bosses (especially Lephantis).
-Zephyr or Titania for Vallis/Plains freeroam and bounties.
-Nekros for farming drops and Kuva.
-Banshee for low-level Defense and Exterminate.
-Gara for high level Defense.
-Gara or Limbo for Mobile Defense.
-Nova or Limbo for Interception.
-Saryn for Sanctuary Onslaught.
-Trinity for Defection.
There's definitely some less common activities (very long survivals, eidelons, orb spiders, etc) with their own best frames and there's
nothing wrong with using any frame for any job, the above is just a safe bet.