I'd say filling out your pokedex is still a top priority for most players.
Mareep (Ampharos) seems really rare so it should probably be in tier 2 or 3.
Heracross probably belongs there (e.g. you go to Disney for a holiday, catch a few [at least one with best moves] then you walk it to power it up).
I would never walk a Rhyhorn/Rhydon. You'll catch them much more consistently there's no need to even evolve the level 20 hatch (you'll find level 25+ Rhyhorns in the wild).