Go to the sources and click on the Total icon for clarification (ignoring legendaries).
Single Click: Pokémon with lowest BST is the first one which is fully evolved when you (single) click it.
Double Click: Pokémon with highest BST is the first one which isn't a Legendary or Mythicals etc when you double click it.
Gen-1:
Highest BST: Arcanine (555)
Lowest BST: Ditto (288)
Source
Gen-2:
Highest: Blissey and Kingdra (540)
Lowest: Smeargle (250)
Source
Gen-3:
Highest: Milotic (540)
Lowest: Shedinja (236)
Source
Gen-4:
Highest: Togekiss (545)
Lowest: Kriketune (384)
Source
Gen-5:
Highest: Archeops (567)
Lowest: Watchog (420)
Source
Gen-6:
Highest (Mega Evolution) - Mega Gyarados (640)
Highest: Florges (552)
Lowest: Vivillon (411)
Source
Gen-7:
Highest: Wishiwashi (School) (620)
Lowest: Wishiwashi (Solo) (175)
(If you don't want Wishiwashi)
Highest: Golisopod (530)
Lowest: Shiinotic (405)
Source
Gen-8:
Highest: Duraludon (535)
Lowest: Pincurchin (435)
Source
Hope this helps :)