Hi "Leonard" - long time no speak.
The other 14?
No 11 is actually one of his more restrained

(and particularly that recording - although now I remember I first heard that from about a floor away at the Heathrow show last year. You could try his 4th, 8th or 10th for something a bit meatier. 10th is generally considered his best - manages to avoid the mind-boggling banalities he normally sticks right next to the most sublime and searing passages. I don't have 4 (the Rattle/CBSO on EMI seems to get good reports), but have excellent versions by Jarvi/SNO on Chandos for 10, and Previn/LSO on EMI for 8 (mine's actually a black bin-lid). I particularly love Previn's blisteringly fast scherzo - inspired by machine-gun fire during the battle of Stalingrad (which pretty much is what this symphony's about). Mravinsky/Leningrad are supposed to be pretty much out in front if you can take the rather dodgy 1960 Russian sound. Titian is the Shosta expert here though.
Others you may like to try in similar vein are:
Prokofiev 5 (Karajan/BPO) and 6 (Jarvi/SNO). Also his ballet "Romeo and Juliet" - Maazel/Cleveland on a Double Decca are wonderful, although the suites give you the highlights in less time.
Stravinsky "Rite of Spring", or "Firebird" for something a bit lighter I recently bought the Dorati/LSO version of Firebird on Mercury Living Presence (1959, UK recording) and it's out of this world - he absolutely transforms a piece that can sometimes seem a bit drab in lesser hands while waiting for the really famous bits at the end - every note is filled with life and fire.
Maybe if Mahler 5 was a bit tame you should try "his" 10th (with a lot of finishing off by Dereck Cooke). Rattle/Berlin gets the plaudits here, although I have a Wigglesworth/BBCNOW off of BBC Music mag that it also superb. The 9th is much more contemplative/valedictory, but probably (along with Das Lied von der Erde) his crowning achievement. If you like the sound of the "LSO Live" series (I'm not convinced myself) there's also the new Jansons/LSO 6 they're all raving about at the moment.
For the really monumental (but less adventurous harmonically) try Bruckner's 4th or 8th (e.g. from Wand/Berlin on RCA), or 7th (Karajan/VPO from 1989?).
Sticking with the slightly atonal you could always try Bernstein's own symphonies. I love the 1st, although there is a mezzo singing verses from the Lamentations of Jeremiah - so maybe not to everyone's taste. There's a good version of 1 and 2, along with the absolutely magical Chichester Psalms, on DG with him in front of the Israel Phil and the Vienna Boys Choir (the LP misses off Symph 2).