most fun: Mindgate, it makes every game different and you sometimes have to guess at what cards you might get from your opponenet
most usefull: thats a toss up between Lobo and Fractal, lobo(or in my case electrocuter) can help you out of many a sticky situation, but fractal can be awesome if you can avoid cluttering your hand.
ones I also like a lot but have not chosen: Nymph/Quintessance - who doesn't like to protect their favourite creature (or your favourite opponents antimattered one)
Twin universe: your mega creature of death had a litter and is now my larger army of death
Dimensional shield: oh you played an overwelming force against me? please give me 18 turns while i sort this out