Basculegion has slightly more coverage than Floatzel has access to (Tera Blast Ghost is strong since despite the Tera reliance it does have the Double-STAB bonus, the power with Jolly is comparable enough to Floatzel's Adamant, and Basculegion's Bulk means it's not as prone to being stopped by neutral priority or as reliant on OHKOing every target to not die.How come Basculegion Male is B rank in the OU Viability Rankings but Floatzel is D tier? Both function as physical Rain sweepers, but Floatzel is significantly faster and only slightly less powerful. Notably, under Rain, Adamant Floatzel's 658 speed lets it outspeed Jolly/Timid Iron Valiant with Booster Energy (546), which Adamant Basculegion can't do at 510 speed.
252+ Atk Dragonite Extreme Speed vs. 0 HP / 4 Def Tera Water Basculegion: 139-164 (36.4 - 43%) -- guaranteed 3HKO (since without Tera he immunes this)
252+ Atk Dragonite Extreme Speed vs. 0 HP / 0 Def Floatzel: 158-187 (50.8 - 60.1%) -- guaranteed 2HKO
Since endgame will usually involve having to get past Kingambit and thus survive a Sucker Punch, it affects what shred of health you need to hang on by.
252+ Atk Supreme Overlord 3 allies fainted Kingambit Sucker Punch vs. 0 HP / 4 Def Tera Water Basculegion: 237-280 (62.2 - 73.4%) -- guaranteed 2HKO
252+ Atk Supreme Overlord 5 allies fainted Kingambit Sucker Punch vs. 0 HP / 4 Def Tera Water Basculegion: 273-322 (71.6 - 84.5%) -- guaranteed 2HKO
252+ Atk Supreme Overlord 3 allies fainted Kingambit Sucker Punch vs. 0 HP / 0 Def Tera Water Floatzel: 271-321 (87.1 - 103.2%) -- 18.8% chance to OHKO
252+ Atk Supreme Overlord 5 allies fainted Kingambit Sucker Punch vs. 0 HP / 0 Def Tera Water Floatzel: 313-369 (100.6 - 118.6%) -- guaranteed OHKO
252+ Atk Supreme Overlord 5 allies fainted Kingambit Sucker Punch vs. 0 HP / 4 Def Tera Water Basculegion: 273-322 (71.6 - 84.5%) -- guaranteed 2HKO
252+ Atk Supreme Overlord 3 allies fainted Kingambit Sucker Punch vs. 0 HP / 0 Def Tera Water Floatzel: 271-321 (87.1 - 103.2%) -- 18.8% chance to OHKO
252+ Atk Supreme Overlord 5 allies fainted Kingambit Sucker Punch vs. 0 HP / 0 Def Tera Water Floatzel: 313-369 (100.6 - 118.6%) -- guaranteed OHKO
This one's an edge case, but being able to take a priority hit with neutral Tera is a very notable trait for a Rain Breaker/Sweeper.