I think if Greenwalt did not leave as showrunner it would have easily surpassed Buffy. But the back end of S3, 4 & 5 are just so different than S1 & 2.
So you have two and a half seasons of one type of Angel show, and two and a half seasons of another Angel show.
I disagree. Tim Minear, for example, wrote tons of season 1 episodes and was largely responsible for the Darla arc (wrote Darla, Reunion, Reprise, Epiphany and others). Even with his contribution to season 3, that season was still awful imo. And Greenwalt was involved in all of season 3 I think? HE wrote Tomorrow which was the finale and a crappy episode.
I don't think Angel's quality is that determined by Greenwalt. Season 4 is about a million times better than season 3. Season 3 can be summed up by Quickening/Lullaby. Loyalty/Sleep Tight/Forgiving, A New World, Benediction, Tomorrow. Everything else is crap and of those episodes, Greenwalt only wrote Sleep Tight and Tomorrow which aren't even the best episodes. The later half of season 3 is the only half I can stand, because it doesn't contain as much dumb!Angel as the first half and actually contains some drama and character development.
Angel season 4 returns to the themes of Angel season 1 and 2: personal happiness vs. responsibility to the world, and forgiveness, redemption and fighting despite adversary circumstances. Season 3 is all about Angel being happy and boring love triangles that don't fit the tone of the show. The Angel team barely works any cases.
I think the only season that doesn't fit in Ats is season 5. That season is terrible overall. To me, Angel's character development ends in season 4. Season 5 is only worth it for Wesley, Illyria, Connor and Not Fade Away.
Also, imo, most seasons of Angel and Buffy are different from each other. I group the high school seasons in the group of seasons I love and are connected/similar. Season 4 is in another pile - different school, Buffy is happier, the gang's dynamic changes, no big arc. Season 5 is in yet another pile - very different from season 4, you can pretty much watch this season and skip season 4. Seasons 6 and 7 are the UPN years and have continuation - similar tone, focus of Spike and Spuffy, etc.
I think Angel easily surpasses Buffy since I only love Buffy seasons 1-3 and have a ton of issues with the rest of the seasons. But that's just me. Regardless, both Buffy and Angel change a lot every season so I don't think that's a criticism you can only make about Angel.