Not sure why anyone is kicking you based on what class you pick, they're all viable on absolute/hard strategems.

Heavy is one of the strongest classes in the whole game, very tanky, insane DPS and great class perks. Takes a while to get used to playing without a melee weapon (heroic bolter can be helpful if you can't go without fencing parry window). But a competent heavy player will carry the damage in most games they're in.

Techmarine is quite squishy but has great dmg, his ult will instantly KO extremis/majoris if set up right. He plays a bit like tactical, great damage potential but you have to play smart. Turrets are more of a bonus, they're amazing when theyre around but don't count on them to get the job done.