I always say that the best system is one that you will actually use. So really, the answer is going to be personal depending on how you work and what is easily understandable for you.
If you prefer a digital solution or are often on the go, something like QuickBooks Solopreneur might work well. It can be accessed from mobile devices and computers.
Some folks prefer paper and keep track of their receipts in file folders and use a paper ledger or Excel file to track amounts.
Any system that works for you is acceptable. The key is that you make the system part of your regular workflow so you actually record everything in it. A complete set of books and records in any format is a much better starting point for your accountant. Four incomplete things because you kept trying different systems and didn't like them will be tougher to compile. Not having one workable system and sticking to it also increases your chances of missing transactions which can cost you at tax time because you won't have all your deductions.
Good luck on your new venture!