The Yak-9U is probably the best plane in the game to help with gunnery as you only get 120 cannon rounds, and that 20mm fires pretty fast so you have to take your shots wisely. This would be a great plane to spend maybe 2 weeks in just so you can have better aim and enhance your performance in other aircraft, like the G-14/K-4 with the 30mm cannon.
For pure fun I'd choose the Yak or KI-84, always great times flying ether of those.