"Best" fish is very subjective. I suppose I'd have to go back many years when I was a match angler and fished with a local juniors club. I got a bit bored of catching small roach and with late teenage distractions of drink, being in a punk band, motorbikes and girls, I started to drift away from fishing. One day I saw an advert in the local rag for the local branch of the Pike Anglers Club advertising for members and I went on a fish-in with them. After a slow day with very few fish coming out, I caught a 12-3 pike that blew me away and one of the members offered to take me fishing on a regular basis. I think I was being groomed as he didn't have a car (or a job!) and I ferried him around for a few years until his first spell in prison - long story, leave it at that.
Summers were spent fishing for barbel with my new found "specimen mentality" and after being single minded on catching a double, after 10 x 9lb+ fish, I smashed it with an 11-14.
When all the barbel in the river seemed to disappear overnight (yes, otters were present), I decided to start doing some Carp fishing on a local day ticket. The first trip was a blank and the second was going that way until some pike anglers packed up mid afternoon and complained they couldn't get a bite because there were too many carp crashing around in the swim. The second they were gone, I decamped and caught 6 with the best being a 19-2 that made me feel that I'd arrived.
My best fish is a 9-6 pike! I hooked it in a narrow river and it went absolutely ballistic! Every time I got it close to the net, it turned, put its shoulders out of the water and pumped it's tail forcing me to back wind until it got back to the middle again. I must have played that fish for a good 15 minutes before I finally netted it but it totally earned my respect.