Echoing the previous comments:
Pumpelly is the longest and best route, nice warm-up in the woods, quick ascent to ridge line, lots of long views well before the summit, limited parking
My go-to loop on Monadnock is similar to one of kerry13's recommendations - Up via White Dot to Cascade Link to Spellman to Pumpelly to Summit & Back via White Dot to Smith Connecting to Bald Rock to Cliff Walk to Lost Farm to Parker
Any of the trails off of Sidefoot to Bald Rock typically provide a short bit of solitude regardless of what is happening on the rest of the mountain. I am partial to Hedgehog.
Hiking in from Gilson Pond is an overlooked option.
When I was a child, the entrance to Spellman from Cascade Link was a bit grown in and almost hidden. That feature added a touch of mystery and wildness to that trail in my young imagination. As a result, Spellman will always be one of my favorite trails.