Excel. I have about 2-1/2 years of running budget that fits on a single screen (1200x1600) w/room to spare. Columns for balance, incomes, "out-goes", savings, extras. Project budgets and other major exceptions get their own sheet within the file. I place indicators at the bottom of columns for recurring expenses like monthly satellite TV bill which goes straight to a credit card. At a glance I know which bills go to what card.
It's simple and works for my needs. I don't use it to track investments or anything like that. For those things I use the sites for the providers.
For what-if planning, I insert a column and populate it to accommodate the scenario. If I don't want to mess with the status quo I'll copy the whole sheet into a new sheet and speculate on that. Right now, for instance, in anticipation of buying a new car I've added a column for the car payment. At a glance I can tell the impact not only in the next month but 2 years down the line. It makes it easy to spot trends like a slowly decreasing balance that indicates too much spending.
Also, it is a living document. Life changes so the planning changes as well. If I have an unexpected expense I see the impact right away. Then I can make adjustments as needed to maintain the balances I desire. Not to mention some things like utility bills can vary wildly from month to month.
Finally, being a simple Excel file it is small, portable, and can be viewed from my smartphone if needed.
BTW, about buying a house, when you budget for that, be sure to include planning for variables like fluctuating property taxes (which tend to rise over time) and higher utility bills. Also, regardless of the home's age, count on spending some money each year for maintenance and/or upgrades. $2-5K/yr is what I use. Sometimes more than enough, sometimes not enough but seems to cover a lot of scenarios like the sudden need to replace the air conditioner or water heater. If not used for repairs, you _will_ find upgrades you want to do. Replace flooring, add or expand the deck/patio, landscape the yard, add solar to the roof, etc.