In case of multiple ILS approaches to one runway, IIRC the "standard" (ie. the go-to) approach is the approach with the letter Z (e.g. ILS24L Z). However, Pilot2ATC seems to always pick the one with the letter Y, which is only the alternative approach (when the standard one doesn't work for some reason). Is this a bug or am I wrong?

There are many reasons why you might have multiple ILS approaches to the same runway and when this occurs, the suffix Z, Y, X etc. are added.  There is no implication of one being preferred over  the other, although the Z approach might usually be the one with lower minimums, it might have either special equipment requirements or a higher than standard climb rate on the missed approach.  So a Y approach is defined with less restrictive equipment requirements and a standard missed approach climb rate.

During flight planning, review the plates and decide which one you would like to fly.

I'll add to my list of future enhancements an option to "prefer" the Z approach, which might be useful if you always fly the latest jets with the latest equipment and high climb rates.

Dave

Thank you for the information!

Dave's correct.  In the US, the FAA's Orders provide for multiple approach indicators when an approach may offer lower landing minima.  The approach with the lowest minima will be assigned the "Z" suffix.  The lower minima may result from additional capabilities, for example, RF legs, RNP-AR qualification, SBAS/WAAS capability to fly LPV or LP minima, or need for a missed approach climb gradient or an RNAV missed approach procedure.  A good example can be seen at RNO for the various ILS approach options for runway 16R.