For some reason I classify
Alan with
Brian,
Colin and Denis/Dennis (the A,B,C,D thing is entirely coincidental!). I think of those names as having the same vintage and all remind me of middle-aged men in knitted cardigans. Not sure why. Having said that,
Alan and
Colin are the two of that quartet that I don't really mind, in contrast to the others.
Alan was brought to England by the Bretons/Normans so it is a well-established historic name. I marginally prefer the
Allan spelling that the Scots favour, and I quite like the Welsh
Alun.
Allen is nice too, but perhaps too much the surname version and too similar to
Ellen.