The strongest one would be Vorse Raider, Gemini Elf, etc. In real life, it would have to be Gene Warped Warwolf, as it has 2000 ATK and no negative effects. The one with the highest ATK is Giant Kozaky, 2500 ATK 4 stars but must have Kozaky on the field.
The strongest monster possible to summon without a tribute would have to be Fusilier Dragon-The Dual Mode Beast with "Skill Drain" active. If you are talking about effect cards, tell me what kind you need and I will tell you. --- Don't look at this Siggy!
You could try the X, Y & Component Monsters (X-head Cannon, Y-Dragon Head & Z-Metal Tank) Individually they may not be the the strongest, but you can combine any two or all three without needing polymerization.
If your looking for Strong 4 star monsters with no negative effects then Luster dragon #1 is good, Vorse raiders better 'cos you can use witch of the black forest to get it in your hand, Gemini elf is the same, But for monsters with negative effects get Spear dragon, very good, cuts defense. Goblin attack force, pretty crap, can't change position after being switched due to it's effect until another turn, spear dragon can. Giant Kozakys really strong, but needs kozaky on field.
Sangan Breaker the Magical Warrior D.D. Warrior Lady Marshmallon Tsukoyomi Elemental Hero Stratos Destiny Hero Disc Commander Destiny Hero Fear Monger Destiny Hero Malicious Destiny Hero Plasma Cyber Dragon Dark Armed Dragon Judgment Dragon Chaos Sorcerer Chaos Emperor Dragon - Envoy of the End Black Luster Soldier - Envoy of the Beginning Witch of the Black Forest
There are many good monsters, but you should generally avoid monsters with a negative or no effect at all unless you are doing a dedicated Normal Deck.