Angel was once one of the most horrific vampires of all time, until he was cursed - with a soul. Now aware and conscious of the atrocities he and vampirekind have committed, he seeks to atone for his crimes. Angel is usually the straight man surrounded by absurdity, but when he does show a sense of humour he can be extremely dry.
A lot of the VAs in the Buffy game sound a little unaccustomed to the transition from screen to voice acting (Anthony Head nailed it though!), and David Boreanaz gets some of that as well. You can hear the spirit of his usual screen performance shine through at times, but overall he unfortunately does sound much more like he's reading a script.
Looking at this purely as a voice compare, my favourite is actually Drew Massey! His performance is a huge part of why Smile Time is one of the show's best episodes ever. His natural speaking voice is actually really close to David's, and he nails the absurdist comedy. Similarly, even though I'm not a big fan of Angel's "Twilight" version, Zach is a talented VA and he does well not only in crafting Angel's new persona, but even letting his more typical spirit shine through at times.
So, looking at things purely from a voice acting perspective, I actually think David's successors carried the torch even better than he did.