Mine, at least until people cotton onto it, is to be a "Slum Landlord." I buy and trade for the cheapest sets I can find, and then I put four houses on each property. Remember, they're slums, so you don't expect hotels. If I can corner the market on houses[1], no matter what the other players do, they can't bankrupt me with rents because they can't improve their properties.
This doesn't work in a game where people ignore the housing shortage rule, of course. But If you toss that rule out, I don't think you're playing Monopoly any more.
[1]: There are 32 houses, so if you own 21 houses, it is impossible for anyone else to upgrade a three property group to hotels. If you own 25 houses, no other player can upgrade any group. It's fairly easy for two players to control the property market and kill the other players off.