I prefer X-Men Last Stand over First Class.
Last Stand at time plays fast and loose with X-Men canon but
First Class pretty much completely ignores it. And while Fassbender is great in everything he does and McAvoy is solid enough everyone else is a boring mish-mash of random and uninteresting characters.
I think the storyline for Last Stand also flows better and makes more sense, particularly when viewed in the light of the first two films.
I wouldn't call either of them great films, nor would I call them bad, but as a lifelong fan of the X-Men, Last Stand >>>>> First Class.
ETA: Although I must add that Hugh Jackman's cameo on First Class was probably the best scene in any of the films. #### off!