Lvl 1 fighter directly multi class into a Fiend Warlock. Now you have a warlock in heavy armor and with 2h fighting style at the cost of a single level.
At lvl 6 (lvl 1 fighter, lvl 5 warlock), you’re an absolute monster at everything.
Also doable with paladin instead of fighter but I don’t really see the point since I already have enough spells as it. I prefer to have an additional action point thx for fighter.