"Strong" is definitely playgroup to playgroup. When in doubt, you can ask your friends what they think makes a deck "Good", and run fewer/none of those things.
To Rose's point, good cards make bad decks good, and generally good cards are expensive. In my fun decks I intentionally play worse versions of staples, either to keep the theme or to keep the deck slow enough so that I don't overstep the table's intended power level.
If I had to just take a few blind shots-
Yavimaya, Cradle of GrowthPerfect Mana makes any deck strong, and getting to completely ignore one of your 3 colors is very, very good.
Burgeoning: 70% of games it does nothing, 30% of games it'll spike you completely out of control to the win.
Consider running
Exploration instead if you absolutely feel the need to play extra lands,
Kodama of The East Tree if you want the same effect but at a fairer amount of mana, or
Court of Bounty if you want the slowest, most situational version
Deflecting SwatFree spells will always feel oppressive. Try
Wild Ricochet /
Ricochet Trap if you really like the retarget effect, or just a regular
counterspell if you really only want the interaction.
Artistic Refusal is funny because you make tokens,
Archmage's Charm has a lot of utility in its' options, and
Cryptic Command is always a good standby
Heroic InterventionIt isn't specifically overpowered, but name-dropping it will get you some looks.
Tamiyo's Safekeeping and
Tyvar's Stand are the fairer single target versions,
Eldrazi Monument is a funny replacement for indestructible that also buffs your Snarmy (Snake Army)
Prowling SerpopardBeing told "You can't do XYZ" is the easiest way, imo to make a deck seem oppressive. I'd
consider replacing it altogether
Purphoros
He's just difficult to remove after he lands on the board. You can run stuff like
Impact Tremors or
Witty Roastmaster since it can actually be destroyed
Kozilek
It's mostly that you can shuffle without expending any resources, imo. I'd
consider Commit//Memory or
Elixir of Immortality so there's an actual
opportunity cost