the romans and persians, both with an awesome UU. my advice is, use the legionairre on higher difficulty levels, and the immortals on lower difficulty levels. why? on lower difficulty levels (regent down) the ai hardly ever counter attacks you, so the extra defense of the legionairre won't help all that much. but the immortals will help attacking. on higher difficulty levels, the legionairre is better because the ai counter attacks all the time. the extra attack of the immortals won't help, it dosen't matter how good the unit is, if they get counter attacked on the way to a city and you don't have enough of them to take out all defenders in one turn, they'll just kill them again on another counter attack.
note that the legionairre is great for victory point games, as they are the best unit for causing attrition towards your opponent, and limiting attrition on your side. (even w/o victory point scoring, this is still great, the ai will give you more stuff for peace the more attrition you cause, even if you didn't take any cities
note that the legionairre is great for victory point games, as they are the best unit for causing attrition towards your opponent, and limiting attrition on your side. (even w/o victory point scoring, this is still great, the ai will give you more stuff for peace the more attrition you cause, even if you didn't take any cities