Never been in the position to have a favorite but I'd say a 69 would be awesome, I could lick her while he fucks her with his bigger cock. Her riding him, so I could lick her asshole and her pussy (his cock too if I could sneak it in). The most loving position, I guess, would be her laying between my legs facing him on her back, me holding her and cupping her breast for him to suck and him fucking her (of course). I could also deal with just sitting beside her and holding her hand while they fucked, that would be nice.