Paizo's "Blood of Angels" splatbook wound up being a lot more fun than I anticipated. (I mean... tieflings, sure. Nothing's cooler than a good guy who's a redeemed bad guy, or someone trying to overcome social censure. But what do you do with a character type all too easily classed as goody-good paladins and clerics?)
Something I thought was really fun were the rules for creating aasimar characters descended from different types of celestials. I'm still really trying to cope with a batch of stuff, so it was a fun thing to work on. Left to right; angel-blooded, agathoin-blooded, a generic aasimar, azata-blooded, archon-blooded.