In regards to @aikohu , this game is obviously class based, so I would recommend trying to Grandmaster everyone if you want to continue to play, which can only take about 3-4 months if you play alot... Once you access a diverse range of characters you'll likely end up figuring out that there is more of a "tier list" with the characters than a best character for a particular style of play or whatever... This game has high tier character, mid range, and a few low... The worst character in the game (in my opinion) was the Scientist just a few days ago, but he just got a prize map upgrade recently which completely modifies the way he can be played... Character upgrades that you get through the prize map are not normal upgrades that you can access through the normal upgrade progression system, and can only be unlocked by either:
1) Getting to the end of a prize map and/or completing the map itself with normal prize bulb acquisition, or you can simply spend lots of money to unlock bulbs and fill in the map that way.
or
2) You can wait until the end of a prize map and purchase the upgrades from Rux.
There's only three prize map upgrades so far, which are:
Rose: Thorn Apart
Kernel Corn: Shogun Guard
Scientist: Steam Blaster
^---All of the above information in mind, my personal tier list is getting towards completion... I only have the Wizard and the Acorn left to Grandmaster, and I will likely have Wizard done today... So here are the characters that I think are high tier so far:
PLANTS/Incomplete Assessment
Peashooter: High Tier/Somewhere in the A Tier range (at least)
Kernel Corn: High Tier/Likely in the S Tier range.
Nightcap: Solid Character/Likely at least A Minus Tier.
Snapdragon: Just finished him today/High Tier/Likely at least A Minus Tier.
Sunflower: High tier/At least in the A Tier range with absolute certainty.
Rose: High Tier/May have been the best character in the game, was in S Tier range, but was nerfed.
ZOMBIES/Incomplete Assessment
Footsoldier: High Tier/One of the best character in the game/Definitely in Super Tier (S Tier) range if used correctly.
80's Action Hero: High Tier/Likely S Tier
Electric Slide: High Tier/Was at least S Minus tier before she got nerfed.
All Star: High Tier/Likely high A Tier; maybe S.
Engineer: High Tier/Maybe S Tier range. He's very good, but he's got so many upgrades that also may be extremely good that I have to study him more to see how broken he may be.
Wizard: Decent Tier and surprisingly solid character. Has some flaws, but if used correctly can actually do big damage from range and has decent abilities, solid rate of fire, etc. At least in the (high mid tier) range, but likely better. Probably in the A bracket.