sigh...please, don't ask about difficulty!
It's all relative, let us know what grade you're at, what have you been working on, and there will be a concerto suited to your skills.
Mozart concerti are top rank, always good to learn, I would recommend any of them (except 8 and 10 unless you have another pianist

). Haydn concerti are not so well known, the dmaj looks to be a fun work.
Play what you feel you can play and what you like - if you like the Greig, play it, doesn't matter in what order you choose.