RCA 5693 is an industrial direct replacement. 6J7 is the predecessor with lass-steep slope and arguably better for audio; my favorite 6J7 is the RCA. 6SJ7 are can be/become quite microphonic. i'd suggest you shock mount the socket with some rubber o-rings or something similar. the 6SJ7GT-MR by tung-sol is a glass version with a mesh plate and they are VERY rare and difficult to shield, but damn they sound nice in a guitar amp, if you can find a working copy at a decent price, you won't be disappointed. next on the favorites list is the 5693 the RCA and national union 6SJ7 are desirable as well.
some say that 6SJ7 is/are similar to the EF86. if i need a pentode front end for a guitar amp - the EF86 is my go-to tube. i
haven't fooled around enough with 5879 to form an opinion, except that they can be very microphonic.
another cool pentode is the EF40, however it has a rim-lock base and they are hardly a mainstream socket, + becoming increasingly difficult to find.
we can't forget the workhorse 6AU6 - a decent tube, but can have too much gain in some circuits. the 6AK5/5654 is another with similar characteristics to the 6AU6, some forum members here have made statements as to liking the 5654 over all.
--pete