I agree that it depends what sort of burger you want as mentioned by others above.
Generally, I like fatty 80/20 ground beef, half beef to half ground pork. To keep it juicy, I dice and fry off some onion and bacon and minced garlic, sometimes finely diced mushrooms, cool then add to mixed meat base. Add enough egg, dried breadcrumbs (you could try soaking some fresh crusts-off bread in milk for 10 minutes, then squeezing out the excess liquid, tear it up and add to mix), some tomato ketchup, seasonings, dried oregano, smoked paprika, ground dried sage. Have just enough sauce and egg in there to make it soft but not sloppy. If it does get a little sloppy, add some plain flour or dried breadcrumbs. If it's too dry, add some more beaten egg or more sauce.
Instead of breadcrumbs, dried stuffing mix in the packet is a good variation, or a packet of powdered French Onion soup. I've also used dried chicken noodle soup - its a nice quick cheat :)
Treat it gently, rolling between wet hands. You won't need flour that way. Chill them down in fridge for 30 minutes before cooking. Seems to help them to stay firm and not crack.
Hint. Always make a little taster meatball and fry that off first before making the rest to check seasonings.
There's so many combinations of meats - pork and prawn, lamb and pork, chicken and pork, all the combos under the sun. Find what you like most, but don't be afraid to experiment. Different herbs, spices, sauces, add grated vegies like zucchini, hide a chunk of good melting cheese in the middle of it. Yum!
When the budget's tight - use packaged sausage meat, just add lots of herbs & spices (curry powder is good), grated veg, bottled sauces to suit. Its also good for using leftover veg in, just chop them up and mix in. While they're not elegant, they fill you up just as well when times are tight.