Been pondering this since you posted it and the trite answer is that it is a tossup between my last peak (Ernest LaForce in Gaspesie Canadian NP) and the next one (Eagle Mtn. MN high point).
Well, those aren't the most challenging nor dramatic of the peaks I favor so I wondered further, what makes a favorite peak for me and I guess I'd break it down into five items: time above treeline, attractive natural features along the way such as ponds, waterfalls, wildlife, a sense of solitude and of being removed from the everyday, challenge and the composition of a larger context e.g who I'm with, where we ate, where we camped or stayed. With that in mind, here are some "favorites".
Baxter Park - Katahdin, especially the Knife Edge, Traveler loop, Doubletop - just love the park ... been going there at least every other year for some 20 years. One trailed peak left of a personal peakbagging mission but many miles left, and visits, of redlining.
Bigelow Range - Maine.
WMNF of NH - Presidential partial traverses, Franconia Ridge, Moosilauke (I was sick starting this hike the first time but I guess the adrenaline cured me of whatever had ailed me!).
Zion NP - Angels Landing, a hike reminiscent of Knife Edge but a totally different environment and, upon reaching the peak, you literally appreciate the namesake.
Tetons and Yellowstone - Our home away from home, though we don't own anything there ... that's reserved for billionaires ... but we have a peakbagging mission of lesser peaks (Hanging Canyon, Blacktail Butte, and Washburn all have very special memories and a visit to this area is always preceded or followed by a foray elsewhere into Montana or Wyoming).
Canadian Maritimes offer adventures on land and at sea and the mountains reach tree line at lower elevations - Jacques Cartier and its small caribou herd in Gaspesie NP, Tablelands and bit of Long Range in Gros Morne NP - so near but so far.
Just bragging ... a sample of favorites!