From a logical perspective, the best one would be Sunflower. We can eliminate a wide variety of choices based entirely on explosive or passive damage potential, namely Cherry Bomb, Potato Mine, Gravedigger, Freeze Shroom, Doom Shroom, Chili Pepper, Spikeweed, and Spikerock. Lily Pad, Magnet Shroom (and Coin Shroom), Lantern Shroom, Plant Pot and Umbrella Shroom all don’t have any orifices either, making sex extremely hard. Torch Wood would burn you alive at the slightest slip-up, and Chomper plant is out for obvious reasons. Eliminating ice-based plants is also a reasonable idea, as contact with their projectiles freezes you, which doesn’t sound ideal. This takes Ice Peashooter and Frozen Melonpult out of the running. Eliminating all the protector plants is a good idea too, as the rigid ness of their skin may be “rough” on your sensitive organs, which means no Wallnut, Tallnut, or Pumpkin. For similar thinking, we can eliminate Cactus as well (his in-game bio literally discussed how he can’t hug most people), and bodily intimacy is important. There’s also 3 plants that are too small to really fuck. This is Coffee Bean, Puffshroom, and Water Puffshroom. Clover, Starfruit, and Threepeater can also be removed on similar grounds. Cob Cannon, in contrast, is too big to fit anything tightly. Lots of plants also sit too low to the ground for proper intimacy, like Hypno Shroom, Cabbage Pult, CornaPult, Melon Pult, and Cattail. Eliminating all peashooters is a good idea as well, since there is a slight yet present chance they may lose control of their pea-shooting abilities during climax. This takes out Peashooter, Double Peashooter, Two-headed Peashooter, and Gatling Gun. For the same idea, all other damaging plants should be removed this far in the elimination, taking out both Fume Shrooms, and Scaredy Shroom. Marigold should also be taken out, as the coins flying around would be funny, but ultimately annoying and distracting. Garlic is shown to have a terrible taste, and since zombies can’t smell, it’s only logical Garlic smells bad too, given the in-game stink lines. Krakenweed also might have complications because of the water (and its subsequent tendencies to drag people down), which poses a safety hazard. Squash would be a solid choice, but ultimately may be too strong for his own good (and yours). This reasoning is a little shaky, but helps get down to one. Sun-Shroom is also good, but has the exact opposite problem: lack of consistent strength, plus you have to wait for it to grow to full size. This leaves Sunflower and Double Sunflower, and since Double sunflower’s heads are smaller and therefore less deep, the ultimate answer is to fuck the Sunflower.