In terms of offense, I think Sawsbuck is definitely a top five Grass type. The other four that are worth mentioning:
- Serperior has good attacking stats and is pretty fast, but it's got a generally shallow movepool as well as being unobtainable if you want a different starter.
- Simisage gets Seed Bomb from the get-go and Acrobatics soon after. It's got other moves through TMs and Tutors, but that strength is also its weakness: you have to invest a lot of time into its movepool. It's also pretty frail and has a marginally harder time KOing things than Sawsbuck.
- Lilligant is a one-trick pony with Own Tempo Petal Dance, but it does that trick very well. Quiver Dance + Petal Dance will tear through favourable matchups, but the only other type of move Lilligant gets is Normal.
- Virizion comes late but has high Attack and Speed, along with deceptively high Special Defense. Swords Dance by level up coupled with Seed Bomb (Tutor) and Sacred Sword (level up) let it rip through a lot of Trainers as long as you mind the 4x Flying weakness.
You might also be able to make a case for Leavanny because of Swords Dance, Leaf Blade, and X-Scissor by level up. I don't think Sunflora, Amoonguss, Maractus, Whimsicott, Ferrothorn, or Tangrowth exert enough offensive pressure to be considered. Roserade's movepool is too shallow -- if you want a one-trick Grass type, Lilligant is almost always better. Leafeon, Tropius, and Carnivine are postgame.