Here's a site you can consult for that info: https://ayumilove.net/raid-shadow-legends-list-of-champions-by-ranking/

Too many to name, but if you're in doubt, go with your starter champion at first - Kael, Elhain, Athel or Galek. All of them are worth taking to 6* level 60, even if all of them would eventually get replaced after you get better options later in the game. This is a game for the long haul, in case you hadn't noticed... 

Some of the good ones you can get for free are:

-Kael, Athel, Galek, or Elhain, depending on which one you picked in the prologue (they're all decent, but Kael is widely regarded as the best, because he has a couple good poison abilities, and those are effective in a lot of places).

-High Khatun (the 30-day login reward).  She's a speed/turn meter booster, and speed is unquestionably the most important stat in the game.

-Ninja (currently available if you log in 7 days in a row, but I'm not sure when they're ending that.  It's soon.)

-Warmaiden (random drop in the "Deadlands" campaign missions) - she has a great AOE drop defence skill, and her affinity (red) is strong against most early game arena teams (which tend to be loaded with blue champs)

-Diabolist (random drop in the "Brimstone Path" campaign missions) - another decent speed booster.

-Armiger (frequently pulled from green shards or for sale in the shop) - kind of hard to build early in the game, but massively powerful once you get his crit rate to 100% and pile on high speed and defence.

-Apothecary (sometimes drops from green shards) - an extremely good, end-game quality speed and turn meter booster, with some healing too, and a multi-hit A1 that's very effective against the Fire Knight.

A lot of champions in this game are complete trash, so Google to check before dumping a lot of resources into anybody.

Some other good ones you'll see dropping randomly from green shards: Shieldguard, Metalshaper, Marked, Soulbound Bowyer, Coffin Smasher, and Avir the Alchemage.

Marked  is  a  Good  hero

While the advice in here is good, this post was approaching a year old. Hopefully OP has upgraded some good champs by this point. :D