Quickbooks imports QBO, IIF, QBJ files. If your Quickbooks older than three years, use IIF format. If your Quickbooks 2013 or later, you may also use QBJ format. Quickbooks Online imports QBO, QFX, OFX, CSV (limited) files.
Quickbooks must not older than three years to import QBO files. QBO (Web Connect) files are imported under Bank Feeds in Quickbooks where they are matched to vendor records and assigned expense/income accounts. QBO files do not carry expense/income accounts (categies) as teh categorization is done after imoort in Quickbooks.
IIF format is considered "low level" or "system" format and Quickbooks imports it right into its database, so you have to be precise with the data you provide. Typos and mimatches will result in new vendor records and accounts created. Always do backup (as with any other import) for Quickbooks company file before any IIF import. Putting aside the scary part, this is great format widely used and you can imoport data quickly. IF you just figuring out Quickbooks, start with the QBO format (see above).
CSV files must have exact vendor names and category (expense/income) account names matching account names in Quickbooks. All Quickbooks versions are supported as long as they import IIF files (except Quickbooks Online). For Quickbooks Enterprise the user must have exclusive access to import IIF files.
QBJ is general journal entry format. This converter is different from other converters. You have to use special Excel template and provide all account details.
Quickbooks Online imports QBO, QFX, OFX, CSV files. Categorization is done after import in Quickbooks Online. CSV files have limited support and have to follow expected layout. Use converters below to create importable CSV/QBO files from your Excel/PDF files.
Quicken for PC imports QFX, QIF files. Quicken for PC older than three years imports QIF files. Quicken for Mac 2015+ imports QFX files and CSV Mint files. Quicken Essentials for Mac import QFX and OFX files. Quicken for Mac 2007 imports QIF files (some users claims QFX files are importable as well, you can try QFX format with the trial and check if it works for your copy of Quicken.
QFX files are Web Connect files and do not carry categories like QIF files. Quicken categorizes transactions after import using the renaming rules or categories can be assigned by the user later.
QIF format is great format supporting categories and tags. Quicken discourages QIF format, but still imports it fine. Quicken 2015 and 2016 have issue with QIF import and QIF files should be imported through proxy cash accounts. Quicken 2017 fixes the issue and imports the same way as Quicken 2014 and earlier versions. Quicken has to be at least Deluxe version to import QIF files directly into bank or credit card accounts. Quicken for Mac 2015 or later do not import QIF files, but Quicken for Mac 2007 does import QIF files fine. QIF Format has many variants, make sure to select correct QIF variant for your Quicken version or other personal finance or acocunting software.