I've thought about this one, and decided I can't limit it to one. There are a handful of mental images I will take with me to the end. In chronological order:
1. Probably a 15" largemouth. I was 9 yrs old and with my Dad on Fontana. I threw a spring lizard into some pollen/trash in the back of a pocket off the Little T. I watched my line go tight, set the hook, and it jumped. I reeled as hard as I could, and the nut holding the handle on the Johnson Century reel I got at my kindergarten graduation came off, and then the handle. Dad ended up hand lining it it for me..
2. A 23" brown on the Little T. I was screwing around waiting on some friends to meet me to fish. I had on a 6X tippet from dry fly fishing on my previous evening down there. I tied on about a size 2 stonefly nymph to mess around. She hit, and ran me into my backing. I wasn't really excited because I was sure she was going to break me off. I started getting nervous when I got her close enough to net. Then, she got tangled in the leader. Finally got her netted. It was my largest trout for a long time.
3. According to my very experienced buddy, a 7-8# smallmouth I hooked on a crankbait. I had her right at the boat and he told me to bring her up so he could net her. He was already telling me I had to mount that fish. As I pulled her up, she jumped and threw the Bagley. He ALMOST caught her in the net on her way down
4. My first tarpon. After dreaming about catching one on the fly for 50+ years, it happened. After being short on my first cast to where the guide told me to throw (I couldn't see her wake), I made a good cast. After a couple of strips, she exploded and ate it as she came out of the water. Her take more than likely prevented me from the dreaded "trout set". It was only around 35#s, but, it was still a tarpon on the fly.
5. Same summer. I had caught about a 90# poon about an hour before. I actually don't remember much about it because all heck broke loose and everything was a blur. My fishing partner was having some back issues and had to sit down for a while, and insisted I try again. The next pod of tarpon looked like they were going to cross at about 50' off the bow. Instead, they turned and headed right towards the skiff. I dumped a cast at about 30' and got to watch this one eat. She totally cleared the water at about 25'. I will take that image to my last breath. She was 100#s of pure energy.
Thanks for the memories!