Try HackSlashMine on hardest difficulty. It creates a sort of class-based Adventure game that involves loads of mobs, dungeons and towers filled with mob-spawners. you start in a low level, however if you want it REALLY hardcore you should make your way to a higher level. being level 1 in a level 4 area will already be really difficult, and as the level rises (of both you and the area), things will get more interesting.

some mods that might help as well: mo' Creatures, Betterdungeons and Battle Towers. the first will create a lod more mobs, including hostile ones. The second will create large towers with 2 spawners on each floor, together with a double chest of treasure. there is a Golem boss on top. this one is very dangerous, as it is tough, fires explosive fireballs, and can destroy blocks.

the last mod adds a large number of structures, all of which are essentially dungeouns. so far I have seen a magician's manor filled with skeleton guards (bearing swords as well as bows, and almost all armored in chainmail or higherm som even in diamond), a volcano filled with dwarves and treaure. this is really big, if you can fine one ot these you can spend ages going through it. it is filled with ores (loads of diamond as well), but the dwarves are hostile (as far as I know), and they sometimes ride giant silverfish which are hostile even in creative mode. at the bottom is a brick maze of rooms with a large number of golems, dwarf spawners, and some other things.
these mods are available in the Yogbox modpacl, but you might want to find another.

both of these modpacks are available in the Techniclauncher. also, if you want to make your own modpack, the new Techniclauncher supports custom modpacks as well. just have a look!

Anyone?