Going back this summer (2021) for the third time.
First year used a Tarptent Moment DW, second year a Durston XMid 1P. Both worked well – XMid is a bit lighter/roomier but the Moment had an easier time finding a spot. Probably bringing the XMid this year as I appreciate the taller headroom.
I have an XMid 2P as well. IMHO would be overkill for 1 adult at Philmont – you don’t take much gear into the tent with you, and you won’t be spending any down days waiting out a rain storm etc. Site selection gets a little harder due to footprint.
I encourage the scouts to use the Philmont tents (MSR Thunder Ridge), unless they want to triple up (i.e. to avoid an odd scout out). Adults use singles, either 1P or small 2P, whatever they have. Our crews both times were large (12 people), needed to crowd a bit in some sites but not bad overall.