Future Trunks' power at the time of his ascension into Super Saiyan was likely lower than that of base Goku's during the Namek Saga, as Future Gohan's power was stifled by the lack of Goku's influence, and Trunks was weaker than him.

No, Lord Slug doesn't count. It's not canon, and it's not even Super Saiyan.

What is the weakest canon appearance of Super Saiyan?

Gohan didn't train during the seven-year timeskip, and had become rusty by the time of the 25th World Martial Arts Tournament, thereby making him slightly weaker than his 10 year-old self.

This question requires some thinking.

What is the weakest canon appearance of Super Saiyan 2?

Implied by both Piccolo and Goku during the Buu Saga, Gotenks' Super Saiyan 1 state was only slightly weaker than Goku's Super Saiyan 3, meaning that with equal transformations, Gotenks' raw power would win easily against Goku.

Longer hair, so longer time to answer.

What is the weakest canon appearance of Super Saiyan 3?

Chiaotzu is retired, Jaco was never a part of the Z Fighters, and Yamcha, Tenshinhan and Krillin are all stronger than Master Roshi.

Post Super Hero Saga, Age 783

Who is the weakest Z Fighter at the current moment in the timeline?

Since Tarble was scared of Frieza's first form, he likely has a battle power far below that of 530,000.

Not counting Neko-Majin Z, sorry Onioheads.

Who is the weakest living, full-blooded Saiyan?

Pan trains. Bulla doesn't. And if you think Goten and Trunks are weaker than Bulla and Pan... I don't know what to say.

Who is the weakest hybrid Saiyan at the current moment in the timeline?

This is proof that the later the saga doesn't always mean the stronger the character. Moori is just as strong as a Namekian elder should be - enough to fly quickly and use magic well, but not much more.

This might be difficult.

Who is the weakest of these Namekians?

When Innocent Buu was split into good and evil, Good Buu ended up being the weaker half due to his intense connection with the Grand Supreme Kai's calming energy. 

Sorry to the Buff Buu, Buutenks and Buucolo lovers.

What is the weakest form of Majin Buu/Boo?

Yamcha is weaker! But not by as much as you might think.

LIGHTNING ROUND #1: Who is weaker?

Hopefully you remembered from Question 2 that Gohan got rusty in the Buu Saga!

LIGHTNING ROUND #2: Who is weaker?

Piccolo told the Z-Fighters not to show up if they didn't think they could at least help against the Androids. And who showed up, ready to fight? Yamcha! That's right, Yamcha was likely far above Frieza's third form by the time of Android #19 & #20's arrival. 

LIGHTNING ROUND #3: Who is weaker?

Despite all the sense in the world, Goku's first foray into the realm of godly transformation is... somehow weaker than Cabba's first appearance. Blame Dragonball Super's powerscaling, not me!

Son Goku (Super Saiyan God, Battle of Gods)

This one's a doozy!

FINAL LIGHTNING ROUND: Who is weaker?

Anime scaling applies here - the manga-only villains are placed based on conjecture.

Who is the weakest Dragon Ball Super villain? (ANIME)

In the manga, Moro couldn't handle the power of the gods, which puts him below everyone else here. In the manga, Fused Zamasu and Lord Beerus still have not been surpassed by any succeeding villain. [ C, B, D, F, E, A ]

Manga scaling applies here!

Who is the weakest Dragon Ball Super villain? (MANGA)

GT Goku got weaker when he transformed into a child.

Who is the weakest villain here listed?

Even Goku knew this wouldn't have worked against the God of Destruction!

Lord Beerus is weaker than a hypothetical Battle of Gods Super Saiyan 3 Vegito.

Who is the weakest Universe 7 Tournament of Power member listed? (PART ONE)

Mostly a process of elimination, but Son Gohan is the weakest here due to being only relative to Super Saiyan Blue Goku. Android 17 fought on the level of Goku's Super Saiyan Blue Kaioken x20 in short bursts, with Frieza and Vegeta being able to do so continuously.