Purely based on speculation on size and "apparent" weight, I'd go with:
1. Huge Ant-Man a.k.a "Giant Man"
Ant-Man has shown that he's capable of increasing his size, and apparent weight, he grows to some ridiculous proportions of > 70 ft tall, hitherto known as "Giant Man". It seems that his apparent weight changes and increases with him. I'd put him at at least the 100 tons range. A behemoth by any measure and a definite winner for our category.
The Hulk/Bruce Banner duality in my eyes should be considered, one. Therefore, at all times, their weight should be technically combined. Now, based on The Hulk's size and bulk, I'd put him at the 200 kg mark, minimum. Couple that with Mark Ruffalo's approximate 1.73 m measurement, I'd put him at roughly 75 kg (similar build to myself). So, he's definitely around the 1–2 ton range when in the Hulkbuster suit.
The following will fit in a similar category and range:
- Tony Stark + Hulkbuster
- Tony Stark + any other suit
- War Machine
"Why don't you make like a tree, and leaf?"
Our old tree friend, Groot, is definitely large, and heavy. I'd put him in a naturally high level, since just walking around he looks and sounds like he's at least 200 kg. Coupled with the ability to extend himself and increase his size (and apparent weight), I'd say he has a huge potential to become one of the heaviest superheroes.
4. Peter Quill?
Ok, I'm gonna start stretching this, since for the most part, I don't think we'll get a more accurate answer*.
Peter Quill is part Celestial, and has access to supposed Celestial powers, namely the white-glowy-power-thingy. Now, as evidenced by Ego in Guardians of the Galaxy: Vol 2, Ego was able to use this power to manipulate the very space and matter around him, to manifest himself as a moon-sized planet, fully-equipped with a shape-shifting body and interplanetary travel ship. Theoretically, and potentially, Quill has the ability to become endlessly big (and heavy). Coupled with his huge ego (lol, yes I did), he's definitely humongous.
why stop there?
Let's see how far we can stretch this thing@
As far as superheroes go, Colossus is one mega-super-ukulele. Able to manipulate his body to become pure metal. Coupled with his massive size, this dude's definitely a contender for this title. Exactly how big and heavy? Well, I'd put him in the 0.5 to 1 ton range. (Based purely on my minimal ability to judge weights).
6. Beast-mode heroes
Look, I'll make a separate category for heroes that will probably be in a similar category that I'll call "The Beast Modes". These are characters that have a natural... bulkiness. Large enough to beat most humans in size-weight ratio, but not quite big enough to make a dent in the floor as they walk:
- Thor wearing, or holding anything
- Cpt. Steve Rogers
- Cpt. Steve Rogers holding, or wearing anything
- T'Challa wearing, or holding anything
- You get the point...
Let's put them at the 100–120 kg range.
* Yes, I'm aware that some smart ass like my mate Valorum will probably come along any minute now and cite a junior novelisation or something, or KutuluMike will come along with screenshots of comic panels. But let's have fun while we can.
@ That's what...
# Hey, you didn't say we couldn't stretch it to the Fox franchise, since Deadpool is somewhat in the middle, I'll happily include this.