So, I'm naming the worst plants ever created in my opinion. Most of them are in PvZ2, yes but just see my reasons.
- Spikerock: Worst in Hist Level (5 out of 10) - In the first game I loved it. Now, it's been debuffed so it will only stand three times as less as it was in the first game? It was perfect when it destroyed 9 rolling objects, but now 3? Why nerf it PopCap/EA?
- Cactus: Worst in Hist Level (7 out of 10) - I don't really see a good point of this character (pun intended). You obtain the Blover on Level 4-4, right after Cacti and Balloon Zombies are introduced. Cacti are only meant to counter Balloon Zombies, but the disappointing thing is their spikes deal the same amount of damage as peas, which is odd. Really pointless plant. Next!
- Bowling Bulb: Worst in Hist level (7 out of 10) - Really how much damage did Wall-nuts do in Wall-nut Bowling? About 20. How much do bulbs do in this game? About 5-8 damage. I actually liked this plant before its nerf, because we could finally get a decent plant in the tough world, Big Wave Beach. But nope. People had to complain about it being OP so even before BWB was released, it was nerfed down very small, and now it's not even powerful anymore. Why, people? Why complain about something that is decent so PopCap and EA won't nerf it? Next time, keep mouths shut when you see an OP plant, no offense, because now you complain about the hard levels and you should regret complaining about the plant. Next!
- Sap-fling: Worst in Hist level (3 out of 10) - So, I thought at first it dealt damage, but it turns out, no, so its cost should be lower. But the projectiles are immune to fire, so it is kind of good in some situations. Plus, the freezing effect of some other plant, makes the zombie go even slower which is cool. I don't have this plant though.
- Sunflower: Worst in Hist level (3 out of 10) - Trust me, this plant is needed, but once you get Sun-shroom and Twin Sunflower in PvZ2, they're useless. It was needed almost any level in PvZ1, and to plant Twin Sunflowers, you needed to plant Sunflowers to do it.
- Gold Leaf: Worst in Hist level (5 out of 10) - It is a Tile Turnip that does increase sun cost, but it can be only be used in Lost City for some reason I don't know. I haven't even got to Lost City, yet, so, I know from what the wiki shows. I agree with Brainulator that is messed up that they made Gold Leaf only usable in Lost City, since you could actually use Tile Turnip in every other world besides Far Future. Ugh..
Plants not included in list
These are ones that I don't hate, but some other people really hate them. Reason why, because they are not really useful but can be useful in a unique way.
- Chomper - I know a lot of people hate this plant in the 2D games, but, I don't hate it. Despite it takes a long time to chew zombies, it can be actually useful in some situations behind defensive plants. Imagine in Big Wave Beach, he can swallow annoying zombies such as Octo Zombies. Then have some other plants for backup to fight the rest of the zombies! Bring Wall-nut and Tall-nut of course, and create plenty of sun.
- Sweet Potato - I know, a lot of people hate her. But despite her high sun cost, she isn't really bad, like what Uselessguy said in some other comments, they can help you out in Endless Zones with Gargantuars. I don't have this plant, but I'm looking forward to buying it soon.
- Starfruit - I know some strategies for it. I don't like the higher sun cost and it being premium, though, but it really can make a difference between some zombies in some situations.
- Split Pea - It can becomes useless later on when you obtain Gloom-shroom and Magnet-shroom in PvZ1, but at least it is better in PvZ2. But then the slowing plants counter the zombie Split Peas are meant to counter.
For the first three plants, use some unique strategies that contain them for goodness sake so you can like them!