@keaton_fox I think it's difficult to suggest just one character for the regions as the quests vary & you may well find it's best to be flexible as you attempt different things. You can respawn any time in the regions if you think another character would be better.
Having said that, for collecting the chests & gnomes scattered all over the regions, characters with jump abilities are particularly useful.
Peashooter for Plants & Soldier for Zombies were the ones I used most, but Corn, Citron, Super Brainz & Imp (with the upgrade allowing the double jump) were all useful as well.
You get a huge amount of xp in the regions, so may want to try a variety of characters anyway - PvE areas are a go-to for most players for getting daily challenges done.
"Fearless" if available for the characters you're using is probably a good upgrade to use, but otherwise just go with whatever load-outs you find best for your own play-style.
There have been lots of posts on the various individual quests & medals, so you'll find tips if you run a search but if you need an odd hand at various points, just let us know in a post & I'm sure one of us will be happy to hop in & give you a hand! 👍😊