Choosing a favorite season of Buffy is tough, because I totally love this show. I will admit that they Jumped The Shark when they brought Buffy back from the dead in Season 6, but that was hardly Joss Whedon's fault. He wanted the show to go for only five seasons and end with Buffy's death. Then the WB sold the show to UPN and they demanded more seasons. Joss has his bosses too, so he agreed, but on the condition that it will be for only two more seasons. That being said, while The Trio were lame nemeses for Buffy in Season 6, The First was pretty damn cool in Season 7 (although it did have a pretty lame name).
But, if I absolutlely MUST choose a favorite season, I'd have to go with Season 3. Just as I thought the women on Buffy couldn't get any hotter than Sarah, Charisma, and Alysin, along comes Eliza Dushku as Faith The Vampire Slayer. Yumm!
As for Angel? I liked that show too. They Jumped The Shark when Angel took over the L.A. offices of Wolfram & Hart, but up until then it totally rocked. Once more I'd have to say that the third season was the best, with Darla giving birth, Connor being whisked away to a demon demention, then coming back fully intent on killing Angel, all good stuff.