Rat Bastard Dungeon Master. In other words, I like to crush my PCs, see them driven before me and hear the lamentations of their players.
When I was DMing 3.5E, I had nearly perfected the art of challenging my big group (7 players, 7 PCs and one major NPC who was run by one of the 7 players) - I threw huge, epic encounters at the group where they went into the battle thinking they were all going to die (and, many of them did, only to get Revivified mid combat...one dwarf fighter died three times in the penultimate encounter)... only to have them barely pull things out in the end thanks to a combination of good rolling on their part, bad rolling on my part, and/or some clever tactics (at the end of the penultimate encounter, they managed to trap the frail high priestess within an antimagic field, where she was basically helpless (STR of 6, CON of 8 without magic) despite her high level because her minions were all gone (not 1 hit point 4E minions). If she had been able to go one more round, she could have Mass Heal'd her fallen allies and likely have won the fight.
However, the challenges I gave them were all above party level in terms of CR.
Many years back, though, I did pull a RBDM move with a beholder in the Underdark that used a cavern that was perfectly suited for its antimagic ray... (why wouldn't a beholder set up shop there if it was a well traveled route?) The beholder then had kobold slaves rain arrows down on the party from elevated & protected positions, and I nearly annhilated the highly magic reliant party with just mundane arrows until one fighter charged the beholder and fought off his troll bodyguards & scored a massive crit on the beholder.