I'm going to go with an all-rounder innings, not saying it is "the best" since it is unlikely we'll find ONE definitive innings above all others, but I think in the context of the match and series it is worthy of mention
Flintoff 102 vs Australia (Trent Bridge 2005)
Background : England had lost the 1st Test badly, a crushing 239 run defeat. England had won at Edgbaston in spite of the aussie last wicket pair adding 59 runs before a disputed catch won the match by two runs. Ponting had saved the aussies at Old Trafford, fighting off England's attempts to go 2-1 up in the series
Trent Bridge : England won the toss and batted. Trescothick and Strauss had put on 105 for the 1st wicket before a mini-collapse saw England stumble to 146/3. England recovered somewhat but, at 241/5 and with Pietersen out, the door was open for the aussies. Flintoff (102 off 132 balls) and Geraint Jones (85) then put on 177 runs and did all but carry England to nearly double their score from 241/5 to 477 all out. Without the injured Simon Jones, and with a dubious decision to enforce the follow on, England still managed to win narrowly. Flintoff added 26 in a low run chase to his 102 and 1/54 and 2/83.
Pietersen’s 158 at the Oval shut out any hope of the aussies levelling the series, but that was on a pitch where the completed innings were 373, 367 and 358 with three other hundreds in the match. Flintoff’s 102 was the only hundred of the Old Trafford Test, Geraint Jones’ 85 was the next highest score and Trescothick’s 65 in the same innings was third highest - and Flintoff wasn’t exactly grinding the runs out. That partnership is arguably what won England the Ashes, sure other innings and efforts made sure they were won, but it was a massive effort in that Test. Had England gone into the final Test 1-2 down or level at 1-1, then it is quite possible they wouldn't have regained the Ashes for the first time in nearly 20 years.
