Q1 - I am only limited to one Ontario fish, it is the Bowfin. Exciting sightfishing opportunities, extremely wary fish, unbelievable attitude and all out fighting ability makes this species my favourite.
Q2 - Caught my first in 2011 and probably caught dozens of them since. (If you have not caught a species, how can you say it is your favourite? What can you base that argument on? Everything is conjecture until you have actually experienced it.)
Q3 - If you are talking about Ontario only, the realistic ones that comes to mind are Least Darter, Slimy Sculpin, Mottled Sculpin, Gizzard Shad, Quillback Carpsucker and River Redhorse.
Otherwise, the list is way too long. There are over 332,000 species of fish and so far I've only caught 577 species. There's still a lot of work to be done. Top destinations at the moment are Peruvian Amazon, Flores Sea, Lake Tanganyika and New Zealand...all of which can realistically offer 30-40 new species to catch within the span of a week.
Q4 - I have too many best fishing memories...577 species...each a great memory in its own right. But if I have to say top 3...
1) First time deep sea fishing with my friend, learning to launch his boat, set out trolling spread, navigate by compass and charts, spotted life on the water, getting a skirt slammed on the pass, 30min fight without any game chair, backing down and steering the boat for the best angle, first time sticking a gaff into a fish, gutting and icing down, docking, scrubbing and cleaning, and fresh sashimi off the cleaning table. This isn't a charter...it's all DIY, all hands on deck, down and dirty fishing on a friend's boat who taught me so much about boating safety, navigation and even some basic boat mechanics the few times we were stuck on the water. Doing all by yourself with first time and landing a Yellowfin Tuna (120lb after chilled on ice overnight)
2) First time speed jigging off a party boat, heaviest jig (14oz) ever used and first time on stand up gear, first hooked and fought a 5' Copper Shark to the leader, then when everyone was having a lunch break and rest, I moved to the bow away from the crowded stern to jig like my life depended on it, finally hooked up and fought the fish without any fighting belt and just a bow rail to brace against, first run took about half the spool and it took 15min after multiple runs to get it up from 130m (400+ ft) of water, and glad it got past all the sharks that were leaving anglers with just the heads from smaller 50-60lb fish. This ended up being the biggest fish of the day, only 1 of 3 landed that day, for the smallest guy on the boat (5'2", 100lb). They all laughed in the morning thinking I couldn't do it, and everyone had new admiration after the smallest guy went the hardest on the 5' shark and the biggest Samsonfish of the trip (88lbs)
3) The first serious attempt to fish for sharks in Florida with proper gear, spend every night on bridges in the Keys sleeping on camping chair, waking up every hour to check the lines and change baits, then finally after a few days the clicked went off, an hour after dusk it was full dark, only one friend was there to help to clear lines, fish went on the upcurrent side of the bridge having full leverage to run underneath the arches, 30min stalemate with the line stretching and rubbing on the concrete ledge, finally outlasted the shark and have it up to the leader, fish caught and counted, attempted to walk the shark 800m back to shore, but shark continued to run under the arches and eventually wore through the 60lb mainline. An hour later, buddy hooked up with another shark, much easier for him to fight this fish downcurrent, but this time there were 3 people including myself to help with spotlight, leadering and rope, finally walked the shark 800m back to the based of the bridge and landed, only to discover my rig still attached to the lower jaw of the shark. Ended up having the chance to take some photographs with my first Nurse Shark (7', estimated 200lbs)