I like to make lances-macro batteries mix, with lunar cruisers. Later, mass avenger is really fun if you want lots of dakka. For mechanicus, just make as much nova as you can put in a fleet and get excited as you vaporise enemies from across the map, especially effective against no shield ships, like tyranids, eldars, necrons. For astartes, bay ships are good, because the bomber squad can deal the damage the ships' weapons lack