Ever since he lost his World Title, he hasn't been really been doing anything other than getting injured. Like fuck, he has talent, we've all seen it, from the dope mic work, to his above average ring skills and yet WWE decides to give him no character. He's just there and the way they use him is they show his past accomplishments rather than creating something more memorable. When he returned to ECW in 2009, he did fairly well, I wouldn't say his BEST work, but he did very well.
After his ECW stint, he did some random shit in 2010 and in 2011 is where he finally caught a break and won his first World Heavyweight Championship. Again, at that year, he did well, but I wouldn't say it was his BEST work, but I'd put it below his ECW stint.
Now his BEST work IMO, was during 2004/2005. He was red hot, he was entertaining, the crowd wanted him as champion, he wanted to be champion, but... the WWE didn't. They thought he was only good as a upper mid-card. Which is why I never understand why they don't utilize him properly. He currently has no fucking character and back then, he had so much.
Also here is him explaining his frustrations with WWE. Around the 6:50 mark.