If you are asking what is the first thing I notice, then its usually the statement piece of the room.
If you are asking what I notice while I'm there, well if you give me enough time I try to notice everything.
lose all of your old ones obviously, if you only had old memories to work with then you'd be fucked, you wouldn't be able to learn anything new or advance in life, and you'd wake up every day being completely confused as to why the world is the way it is. whereas if you were able to make new memories you'd eventually figure out where you were and what was happening and you could get back to the place you were or even go farther than that.
what is your greatest fear
Either way, you essentially die. If you can't make new memories, then you are permanently stuck in single state that you relive over, and over, and over again, but never know. What your life used to be is gone, and you'll either live day to day confused and fucked to where your life went, or everybody plays pretend and the rest of your life is an illusion. I would personally prefer the latter, despite the fact that I would not exist anymore. Without your old memories, you are are a completely different person. You are very alike to who you used to be, but you will both be completely separate people forever.
tl;dr I pick the second one