The $26 burger at Tavo is overloaded with toppings, including a portobello mushroom hidden under the bun.Gabi PorterBurgers are bigger, brawnier and more omnipresent than ever, boasting exotic add-ons, “proprietary” beef blends — and higher price tags. But it doesn’t mean they’re necessarily better than cheap, old-school, diner-style ground-chuck numbers. That’s especially true when a kitchen dolls up lame, Wagyu-blend patties with a salad bar’s worth of incompatible toppings that include cheap foie gras, portobello mushrooms, avocado and alfalfa sprouts, as at new Latin-fusion Tavo on Hudson Street. I grew up loving simple patties grilled to a succulent turn, like the $10.95 “classic cheese” at several Burger Heaven locations and the $9.42 cheeseburger at the Parker-Meridien hotel’s Burger Joint. Both are demure enough to be eaten by hand and immensely satisfying. By comparison, most of the new crop of mega-burgers — made with designer beef blends, topped with the kitchen sink and costing small fortunes — fall short. The weaker ones range from meh (some flavor but lacking in moisture) to miserable (neither flavor nor juice). The best are just too damn expensive. None could be comfortably hand-held and a few might best be consumed with a forklift. THE CHUMLEY BURGER $25: Chowhounds are buzzing about the double-patty at the new Chumley’s (86 Bedford St.; 212-675-2081). Too bad the beef is totally bland.Gabi PorterMost of the new breed rely on baroque, custom-tailored blends of chuck, short ribs and brisket, with wildly varying results. The year’s most celebrated new burger is…more detail