0182 Bellossom

Razor Leaf features incredible damage output with STAB, at the cost of having the worst energy generation in the game. Bullet Seed is the exact opposite, trading away damage output for significantly better energy gains. Either option can be viable. 

Leaf Blade is one of the best charge moves in the game due to its low energy cost and high damage value, and also gets STAB. Dazzling Gleam and Return both cost a lot of energy, though they offer different typing coverage. Petal Blizzard is inferior to Leaf Blade. 

As a mono-Grass type contender, Bellossom has more weaknesses than resistances, but this might not always be a bad thing as it avoids some of the problems that its Grass/Poison friends might have, mainly against Psychic-types. While Bellossom benefits from decent bulk thanks to a relatively lower ATK stat, its offensive power is actually its main strength thanks to its incredible moveset. Access to Leaf Blade and two strong fast move options gives it a lot of potential to deal massive amounts of damage. Overall a good pick for Great League, though Bellossom does lose some points due to big losses against Tropius and Venusaur who fight for the Grass-type role. A lack of adequate coverage moves also can leave Bellossom very vulnerable. 

The level 50 update has improved Bellossom's performance in Ultra League, and challenges much of the meta thanks to its fast Leaf Blade spam. Bullet Seed is generally preferable as it increases accessibility to Dazzling Gleam to threaten popular Dragon-types. Unfortunately the lack of other coverage options causes it to often lose to other Grass-types in this league. 

I guess Bellossom gets some credit for wins against Swampert, Kyogore, and Groudon. But yeah, consider other options.