I have a couple favorite boss fights, all strategy.
First let's start off with, Last boss in HoS, mann that boss was a pain at level 55, yet alone getting to it, stupid guardians, now it's nothing.
Speaking of getting to him, Syphodex was a pain too, the chariot boss, but was easy once you knew how to do it, jsut the randoms tun sucked butt.
The first boss in DL was a pain but was so fun to kill doing the strategy, kiting him while he spinned, sigh good times.
My next favorite boss fight Was Paicha. Palenque had issues with people moving to Rift during Chapter three, people couldn't clear RT. That's when the guild I was in, Kealani's guild Benevolent broke up, mann that guy could coordinate a seige. After Benev broke up I moved with Noony and Draden to a little guild called Notorious, little did we know we'd help change that guild forever. RIP Katie. The guild rivalry with Sins was also fun, beating them to the punch was the best

But Paicha was the hardest boss, we didnt have enough DPS to burn it like other servers, why're there bubbles on us? why the debuffs why cant we cast or move? figuring out that boss was so fun and helping MA's healers learn that boss was also fun, ( Scarfury, Vavdichal)
The next boss is the Tree boss in RT. that boss was a pain for us for quite some time as well. Especially with the stupid balloons draining mana. WE had to get desert bow's from KT first before we could continue. once we did RT was easy. once we beat Rt bronze we cleared dia. But those are probably the two toughest bosses in that instance.
Next boss, Well it's hard to pick just one so pretty much all of GCH. That was my favorite instance. Probably still is. After chapter 3 we had a fallout of scouts, I moved onto MA. mann Intox could lead a guild. Best tank I've EVER seen or been with hands down he could tank and hold aggro with ANY tank, all knight classes and warden/warrior. the infamous s/wd m/w w/k r/k r/s the dps hogs. Once we killed Annelia, after lots and lots and lots of deaths and tries and hours of mages kiting. we finally got our runs down to45 minutes each run we could run like 2-4 times a night. But theres so much going on during the Annelia fight, it's amazing, you HAVE to strat or no bueno.
Lastly, the Tomb of Seven pheonix's err, Heros

Loe the mechanics and the strats even if they were super buggy. first boss, second boss, sheilds! oil!!! and 4th boss, rest were pretty much cake but those three were always fun to go die on ;p
Those are my favorite boss fights...