You know, I don't think I'd put Thrawn at number 1, and would keep him below Ackbar.
Consider- he brilliant plucks winning strategies out of the air while examining people's art. Now, I'm sure his head is just full of tactics and battleplans while doing so, but to the outside, including his captains and commanders, it's semi-mystic. Not something they can learn from.
Thus Thrawn is a brilliant tactician and strategist, but he doesn't pass that on to anyone, and indeed, he overrules his subordinates enough that it can be a reflex to trust Thrawn's judgement rather than their own. So in the end, when he dies, the battle falls apart. Even Pallaeon, Thrawn's closest, wasn't trained to fill in the gaps and independently come to a result that, while perhaps not as good, might've done the job. Pallaeon learned some, but not as much as a guy as smart as him could've.
Compare to Ackbar, who doesn't have quite as good a spark of brilliance, but will point out what he thinks the enemy is going to do and why on a map. The result, a lot of good commanders who learned from a great commander. When he leaves, the Republic can still manage without him much better, and we see that with the Republic's rather good fleet officers in later wars.
I also wonder how well Thrawn would've done on a much larger scale. His Remnant was in a size that he could oversee many of the important battles personally, but when you get larger, like if he'd absorbed the other warlords, you have to let other people handle other fronts and even make strategic decisions. He might've found himself in a position where wherever Thrawn is is doing great, but everywhere else is faltering because the lower commanders aren't as good as the ones who learned under Ackbar, because when the going get tough, their instinct is to look to Thrawn.
Just as Emperor Palpatine made himself a political linchpin, so too was Thrawn a different type of linchpin. While it helps him pull off his magic, it's a liability.
And then I'll toss in both Revan and Gar Stazi were 'teach the subordinates how to do their thing' types too. Revan's Empire obviously kept trucking without him, his commanders had learned well, and we see Stazi's subordinates pick up on his tactics when he was disabled (and indeed, he considers them quite able to carry on if he were to fall).