Bellossom for
B or C rank
Due to its versatility that it can offer
As Attacker
Bellossom@Weakness policy
There are 2 ways to use bellossom with WP
1.Special attack
Nature: calm
EV: 252HP/96SPATK/160SPDEF
-sleep powder
-giga drain
-hidden rock/fire/ice
-moonblast/synthesis
This set is my classic set. Due to bellossom's bulkiness, it can survive most ohko from SE moves from unbuff opponent which will activate weakness policy and sleep the foes and counter them back with your boost power. This strategy do not need support from teammates and can counter back foes most of the time. However this set usefulness will down if bellossom did not hit by SE move and spdef wall and pokemon that resist all its move can wall it. The EVs is designed to sustain sp.atk from the like of unbuff flamthrower chandelure, dont use it to take SE hit on physcal side.
2.Physical attack
Nature: Adamant
EV: 252 HP/160 ATK/20 DEF/ 76 SPDEF
-sleep powder
-drain punch
-petal blizzard/leaf blade
-sword dance
Well this is very classic physical set that most of people know it and yes it can work better with WP free boost and more boost from SD. This set if play wisely it can punch through most of the thing however it will be useless against bug, poison, flying and sometimes on...fire and BURN status but it did work and save me many matches and some learn it the hard way. the alternative of this set is hold sitrus berry instead of WP to give you instant heal and sleep the enemy or scope len to boost crit chance if you use leafblade. It also not need much support from the team also.
*note Life orb and Assault vest can be use with bellossom too, especially AV that increase its decent spdef so you can max out spatk and hp.
*Other classic standard bellossom's atk/spatk sets, e.g. sunnyday chlorophyll combo set, life orb spatker, etc, can still be use too but need to play wisely.
As Wall/Support
Bellossom@Leftover/Rocky helmet/WP
Nature: Bold
EV: 252 HP/200 DEF/56 SPDEF or 252 HP/252 SPDEF/4 SPDEF
-sleep powder/stun spore
-moonblast
-synthesis
-giga drain/leech seed
or
-stun spore
-sleep powder
-hidden power rock/fire
-giga drain
*For special wall flip the EV and nature to calm instead
Thanks to def boost, bellossom can become physical wall to an extend with full ev def@bold can make it able to survive choice band talonflame sometimes and cripple it with status. rocky helmet can be really annoying for physical fighters and moon blast, giga drain/leech seed and hidden power can provide good coverage so these is kinda mix between wall and support and can be use to annoy anyone who underestimate it.
Conclusion
There a lot of things you can try with bellossom, its stat give it quite a versatility if you use it correctly. There are 2-3 more sets that I had tried but I will save those for later. This thing is now that useless if you using it and it need little support from the team to operate but still some set will be better with A rank class that why I support bellossom to be in
B rank because of its versatility and need less support to be use and if not, C rank is also plausible for it. Well I hope you guys consider my opinion. Thanks for reading this.