There are numerous other birds that could be put on this list, but in terms of rarity and/or personal significance, these seem like the best choices for a top 5 (I've included the date of the sighting for each bird):
1. Black-bellied Whistling-Duck - first LA County record, seen at Bonelli Regional Park (7/1)
2. Trumpeter Swan (lifer) - likely the same 4 birds seen twice on separate days in Hayden Valley, Yellowstone National Park (7/28)
3. Couch's Kingbird - second California record, at Sunset Park in Visalia, Tulare County (11/21)
4. Sinaloa Wren (lifer) - long-staying bird at Fort Huachuca, my rarest bird of the year, and my first sighting of an ABA Code 5 bird (11/23)
5. Yellow-eyed Junco (lifer) - lifer #500, seen late in the afternoon at Miller Canyon in the Huachucas (11/23)
Honorable mentions:
1. McCown's Longspur - kicked off 2015 with a lifer on New Year's Day! (1/1)
2. Painted Bunting (lifer) - female that showed up at a friend's feeder, just a quick bus ride from home (3/30)
3. Scissor-tailed Flycatcher - 2 different male birds: summering bird in Ventura County, nesting with Western Kingbirds (6/21); and wintering bird at Woodlawn Cemetery in Santa Monica (12/12)
4. Neotropic Cormorant - first Ventura County record, new for my California list, and like the Scissor-tailed my first time seeing the species since my Texas trip in 2010 (6/21)
5. Hooded Warbler (lifer) - male at Huntington Central Park in Orange County (6/27)
6. Common Nighthawk (lifer) - distant bird seen at dusk from our hotel balcony in Beaver, Utah en route to Yellowstone (7/25)
7. Harlequin Duck - first time seeing the species in nesting habitat, at LeHardy Rapids in Yellowstone NP (7/28)
8. American Oystercatcher (lifer) - seen on jetties in Newport Beach (9/25)
9. Brown Booby - 80+ on a Channel Islands Pelagic (10/10), then 1 lone bird on the rocks at La Jolla Cove in San Diego (12/27)
10. Chestnut-sided Warbler - by far the rarest bird ever to show up in my own yard (10/18)
11. Arizona Woodpecker (lifer) - one of my most-wanted birds from my inaugural trip to southeastern Arizona, woodpeckers being one of my favorite groups (11/23)
12. Northern Cardinal - a single female bird in a yard in Ajo, AZ, which we staked out for a reported Rufous-backed Robin (which didn't show); first time seeing the species since Texas in 2010 (11/24)
13. Crissal Thrasher (lifer) - seen at Yuma West Wetlands on our last morning in Arizona (11/25)
14. Little Blue Heron - distant adult bird seen in marsh at Tijuana Slough NWR; again, first time seeing the species since Texas in 2010 (12/27)