Avenger isn't great, but is a fine filler card in low-CMC lists with a lot of mana dumps. Just an efficient guy to play off-curve.
I like 5-color decks, and mael angel is certainly a powerful reward. Very answer-this-or-lose, though.
I've actually never seen Guardian Seraph, very different wings than the generic angel deal. Seems fine for low-CMC cubes, but when every color has access to beef it's pretty bleh.
I have a mild bias for basandra (had a fun "lash your guy, but I wouldn't attack ME if I were you" multiplayer deck) but her abilities just aren't great 1v1.
I like this one for various reasons. Probably the lowest-power angel I'd play: