My most prized fish is my clown pleco, Willow.
I estimate I got Willow sometime before I entered high school and since I'm 37 now, that makes him among the fish I've had longest along with his "wives". He's my favorite because I could pick him out easily with his "hairy" tail when he grew up.
Willow cost me $4 at the LFS because they were having a buy one get one free sale and clown plecos were $8 each.
I had Willow in my 20 gal long community tank I got for Christmas which was my first non noob( or less noob) tank since upgrading from a 10 gal. Willow's tankmates were a long series of bettas, cherry barbs, rosy barbs, tiger barbs, cardinal tetras and his female counterparts.
Willow lived in that tank for many years before eventually moving into my 55 gal community tank. Willows tankmates then became jurapari cichlids, rosy barbs and tiger barbs. I took that tank to college and kept it in a dorm .
When these tankmates past, Willow remained alone with his female counterparts.
This is where Willow is currently and I'm debating whether to end his boarding experience with my parents and bring him and his "wives" to my new rental house I moved into last Dec.
If I pick my most rare fish, would have to have been my blue eyed pleco, Ginny. I still count among the worst fish losses...But I didn't have anyone to take her after I moved away from my parents house and boarding life was too hard on her.
Most expensive would have to be one of my mail order HM bettas. The kind that seem inbred with shortened lifespans that I don't buy anymore.
Probably soon my most expensive fish will become my Rain Garden wataoni that I plan to order this spring.