My entire game is pretty much houserules so it's hard to come up with any particular one that's a favourite.
Houserule I dislike the most: any blanket rule against otherwise-reasonable and-or generally entertaining character types or activities that would otherwise be present and accepted within the setting. Some examples of what I mean that I've seen on this forum (!) over the years:
--- no evil characters
--- no chaotic characters
--- a ban on playing characters of a gender not your own
--- no in-party romances, flirtations, dalliances, etc.
--- no henches, adventuring hirelings, or secondary characters
--- no outright disagreement with another PC's suggestion or idea, no matter how dumb it is
Note that these are all different from setting-based houserules e.g. if a setting has no Elves in it then a houserule saying you can't play an Elf is fully justified. But every setting has evil and chaos in it, every setting has (at least!) two genders, every setting has romance, and so forth...and therefore it should all be playable.