This tier list does not rank them by their raw power, but rather than thier viabilities or worth investing resources in. Positions within ranks do matter ( I think sun-shroom are better than all it's peers in S rank for example). I put some t1 and t2 above other t3 not because they are "stronger" but their abilty upgrade significantly buff their t2 t3 evolution, hold on good enough for you to farm suns at the start or you don't have to immediately merge them to unlock potential. I've gone pretty deep into expedition now and beaten 2 lawn patrol on hard after the revam and I'll briefly explain my ranking:

\-S rank: goated, never regret upgrading them, hold on very good against overleveled zombies and will get brought into most stages.  
I put sun-shroom t1 in because it is the only case where the ability upgrade (parting gifts) VASTLY boost the their t2 t3 (in this case heavily improve sun production and suit their debuff due to disappearing in sunlights).

\-A rank: pivotal, sure they get overwhelmed by zombies pretty quick but still very reliable as support plants or sub-dps during mid-stages onward when the amount of suns is abundant and spaming plants is crucial for a 3 stars win. Some of them open paths to other merges across plants so leveling them is worth it.

\-B tier: either merge as soon as possible, outclass by other option or very situational. Tangle kelps family is very good but so inconsistant, I needed them to get 3 stars in stage 10 campground expedition, took alot of tries but eventualy got it.

\-C tier: they are fun to play with in easier level but definitely not worth upgrading.

\-D tier: disappointing.

The reason I haven't unlocked all the plants because I don't think I need them. I still have 600+ wild seeds (idk why but I like hoarding them) and I am focusing on farming peashooter for level 5 t3 ( next will be cabbage, lightning). I would love to debate or discuss about each individual plants if you would beg a differ in the comments because I can't rank every single plants haha.

My Pvz3's Plant Tier List