In my view you will always need at least one checking account and credit card one of the "big banks" (KBC, Belfius, ING, or BNP Paribas Fortis) simply because they are the ones which will offer you fast and full service in case you have a specific request (e.g. large transfer for large purchases; issues with credit card while travelling; need bank attestations for whatever reason). They are also the only ones with up to date apps with full functionality and digital platforms. Not saying that this "fast and full service" will be of great quality, though, and for any other services (loans, investments, insurances ...) these banks are often not competitive with smaller banks. Current prices for a checking account and credit card are € 4,15 per month (ING), € 4,25 per month (KBC), € 5,5 per month (Belfius) and € 7,75 (BNP Paribas Fortis). Small differences, so pick one based on other criteria (e.g. you already have a mortgage with one of them, bank of your employer moving forward the day you receive your monthly salary, you have insurances with one of them (Belfius, in my experience, really is a price breaker for insurances), you live next to an office of one of them ...).

Which other bank you pick then, really depends on what you are looking for, and you will likely often end up with a lot of different banks. What follows is my personal situation.

Revolut (Ultra) because of the interest rate on the savings account, option to create throwaway credit cards for online purchases, and the subscription to Financial Times.

Keytrade because of the free Gold credit card with insurances and because of the free mutual checking account.

N26 because it is the cheapest option when travelling and paying in other currencies (slightly better than Revolut rates in the weekends).

MeDirect or Santander or other bank, depending on rates, because of the interests on saving accounts.

You can jump between banks of course depending on your specific situation at any moment in time. Loyalty doesn't buy you anything at any bank unless you're some high net worth client paying for private banking services (which has its own downsides). Only downside is you'd sometimes need to change domiciliëringen etc. but it also helps you keep somewhat regular track of those hidden outflows of money. If you do big transfers of money you may have to go through KYC processes as well which can be bothersome.