All based on your personal preference.
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Excluding custom heroes, captain mercenary unit is the most effective one after you have income to buy weapons.
If you want to focus no Nightmare, Air, Invis, 4X, then there's no obvious difference regarding choice of heroes when your focus is to kill enemy. (You should focus on lumber to survive)
In short, if you are lazy, use DEX type heroes or Commander. (upgrade Adrenaline and Critical Shot, or just call in paratroopers regularly) In time of need, DEX and STR heroes can take the role of tanks to defend Air bosses.
If you need more ability (also as life saver) to fumble about, use INT type heroes. Atomic Surge is just op against 1X boss. Death waves/Super nova is the most effective AOE ability with advantage of cast time, cast range, CD time, and extra buff.
You won't see much difference if you play 4X mode on higher difficulties, because Neither type can kill bosses + units fast enough, that means you still have to focus on lumber so your walls won't fail you.
If you can survive without stalling, and you want to kill enemy faster, at that point any hero works for you.
If you stall, there's no point to use hero to fight, when you are ready to clear the field or to kill last bosses, you don't even need a hero.
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I'm on rank 70, and I still use Alchemist and Scientist more than any other heroes. For your rank, I would recommend you to use Astronomer, if you are tired of Commander. If you like fighter/DEX type heroes, I would suggest you to keep using Dark Sniper. I like the chaos damage more than multiple-shot of Windrunner.