It depends a lot on what you want out of a POS. Are things like inventory management important? Do you want to run reports from within the program? Do you need it to do accounting/timesheets/staff pay? Do you need it to take credit/debit payments for you or are you going to get your own machine? Do you want a piece of software that will also track repairs?
Lightspeed or Ascend are generally the two top things (More than just POS) that for profit bike shops use. Square is pretty common as a basic POS/credit card processor at bike co-ops from what I've seen. There's also Clover which seems close to Square but I haven't touched that before. Quickbooks is great for accounting/running reports but is horrible as a POS.