Ulysses, oooohhhhhhh!! Ulysses is just so fantastic to me. Being one of three fantastic stars of Lonesome Road and constantly mentioned throughout the other DLCs, I had very high expectations for his character... and he exceeded all of them.
His strange stalker-ish obsession with the Courier and profound historical comprehension work together incredibly well with his Woobie, Destroyer of Worlds actions and witnessing that is just simply incredible. But then, talking him out of his crazy plan and fighting side by side with him after all of that criticism and philosophy was just the perfect ending to an otherwise 'meh' DLC. His sheer power, cool mask, and awesome voice with those fragmented sentences really supported that climactic confrontation at his temple, altogether cementing him as even one of my favorite characters in any fiction (partly due to being a total escapist character to me). I can never resist to urge to recruit him with mods just to fight side by side once again.
Almost as much though, I like House. He's a totally cool insufferable genius that can be a bit off-putting, but I just can't help but be charmed by him. On my first playthrough, I almost betrayed him to take Vegas for myself, thinking I could do a slightly better job, but after reading his obituary and hearing his speech about his plans turned my opinion from just "I kinda like him," to full blown admiration and a deep desire to support his Vegas. What I like the most about him though, is how rewarding it feels to be complimented by him. He's such a smug jerkass most of the time, when he expresses how impressed he is by the Courier, it's genuinely feels more rewarding than any other achievement in New Vegas. It certainly helps to have shared his political ideals before even meeting him.