Lol the amount of "fav X" threads can only grow, I love it
This is quite an interesting category in the way that most of the options are grass types, and grass has a lot of pokémon I like.
For flower: I love Lilligant, Cherrim, Roselia/Roserade, Jumpluff and Florges, but my fav hands down is Vileplume. Oddish is one of the most iconic grass types in my mind, and it gets better when it gets to vileplume. I'm sad they don't get leech seed tho, except for an event one. I also like the side-evolution of this line, Bellossom, although I preffer Vileplume over it.
Mushroom: has to be Breloom, although I love Parasect and Amoonguss as well. Breloom can hit so hard and it is the best offensive spore user out of the 3, while Amoonguss bests it on defensive use with that god-like regenerator and the bulk. Parasect is fragile but can do some appreciated stuff in game.
Cactus: Maractus. Is just too cute. I also like Cacturne a lot, but Maractus has the cute factor on its favor.
Tree: I like them all, mainly Exeggcutor and Trevenant (the last being my fav out of the 2). Exegg can do some amazing sweeps with harvest, and Trevenant's harvest allows for VERY annoying sets instead (substitute + w-o-w/ + eech seed + horn leech/phantom force + sitrus berry, for example). It bests Exegg on having a ghost/grass typing that only Gourgeist shares. Gourgeist is my fav mon as well but is not a tree but a vegetable rather, so it doesn't fit in any of these categories.