I don't like any of the rome expansions to be entirely honest, just the vanilla.
I have Barb invasion, but I find it very hard and thus not that fun.
Alexander, I don't own it but I have tried it on release, it was also quite hard but I couldn't be bothered to play it longer than bout 30 minutes so I can't really tell.
I'd say buy the gold edition for a fiver ( in the NL several shops sell it for 5€, rome + barb invasion) and after that if you get bored of them try alexander.
Many people also swear by the total realism mod for rome, I can't say I've tried it though.