Wolfram & Hart
While it's not exactly clear when Hamilton, it is clear that he was devoted to them with his first appearance (17x5, Angel, Underneath) when Angel and Spike racked down Eve only to find that the Senior Partners had some how divined her location despite painstaking spells placed by Lindsey McDonald. They'd send Hamilton after her, though at the time, his intent was unclear as he'd kicked down the door easily only to find his quarry had escaped.
Of course he effortlessly followed the group to the offices of Wolfram & Hart. While Angel, Spike and Gunn had absconded to a hell dimension in order to save Eve's lover Lindsey to pump him for information, he'd left Eve under the protection of Lorn and his vampire assistant Harmony. While a security guard attempted to keep him from entering Angel's office, Hamilton quickly dealt with him - by punching a hole straight through the mans chest. This of course was met with screams by the team supposed to protect Eve. Harmony too tried to distract him as Eve and Lorne took the elevator in Angels office down to the parking lot. Once again, resistance seemed futile as Marcus easily incapacitated her simply by throwing her across the room.
Down in the garage, Lorne and Eve met up with a returning Spike and Angel, with Lindsey in tow. Gunn unfortunately sacrificed himself to save Lindsey, trying to atone for the situation he'd caused with Illyria
. Of course once again, Hamilton managed to find the party. But when he confronted Angel, his motives became clear. He wasn't out to hurt Eve. He'd been appointed as the new liaison to the Senior Partners, as they'd obviously felt Eve was easily manipulated, and had of course betrayed the Partners. He forced her to sign over her immortality and all of the benefits of becoming one of the Senior Partners instruments on earth.
Not Fade Away
With Angel seemingly taking a much darker path than anyone could have expected, Hamilton became a close adviser to him as Angel attempted to join the Circle of the Black Thorn
. He instructed Angel on the right members to approach, presumably how joining would occur, and even apprehended Drogyn, the Battle Brand for Angel's initiation ceremony, going through even Illyria to do so. He and the rest of the Circle forced Angel in to a corner, getting him to sign away the Shanshu prophecy.
Eventually, Angel's plans came to fruition, as he and the rest of his team started slaughtering members of the Circle wholesale. Presumably, it was Angel's job to deal with Archduke Sebassis, though when Hamilton confronted him before he left, Angel revealed that the Archduke wasn't his target at all. Earlier, he'd spiked the Archdukes 'drink' and killed him. Hamilton was his target, and a fight ensued between the two. Even with Angels enhanced abilities, Hamilton gained the upper hand with no problem whatsoever. But when he had Angel on his back, about to deliver the finishing blow, Angel's son Connor
intervened, and once again the fight continued. But despite the duos enhanced abilities, Hamilton was demolishing them. He knocked out Connor by tossing him in to an elevator door, and was once again nearly killing Angel. But in a stroke of brilliance, Angel realized that by drinking Hamiltons blood, he could increase his powers. Doing so, he ultimately defeated Marcus, which drove the Senior Partners to unleash an army upon L.A.
Hamilton was easily one of the more powerful beings in the entire Buffy and Angel series. He had super strength on a massive level, allowing him to contend with and defeat even the most skilled of vampires, which he proved by essentially dominating Angel during the only fight they had in the series. His strength and skill even allowed him to fight and defeat Illyria (whose powers had been reduced by the Mutari Generator however.) However, she was still capable of taking out Spike nearly every time they fought, and killed demons that gave him trouble. He was even capable of fighting two enhanced beings at once when he fought both Connor and Angel, and still received little to no damage.
Hamilton also possessed a psychic uplink to the Senior Partners after replacing Eve, and he also presumably gained her immortality despite already obviously being quite long lived.
However, Hamilton's great strength is also his only weakness, as his Blood is infused with the power of the Senior Partners, this power can be stolen from him, as was seen when Angel drank his blood, and consequently snapped Hamilton's neck.