For me, there are a few.
Bobby Lashley
I remember seeing his debut match and I was amazed by his size and power. He may not have made it as an absolute megastar because of his lack of charisma and then leaving the business, but when I saw him in the ring for the 1st time and how dominant he looked, and knowing Vince's hard-on for bodybuilders, I knew Lashley was going to get a major push
John Cena
To be booked as strongly as he was against Kurt Angle in his debut showed me that the WWE had BIG plans for Cena. He looked good, he could hang in their with the best in the business and not many rookies get that much offense in on their debut against a top star. Cena seems to have got worse in the ring since then, but on his 1st appearance he already looked like a star.
Brock Lesnar
He just looked incredible. He destroyed Al Snow, Spike Dudley etc and I had never seen anyone deliver a powerbomb in the way he did to Spike, picking him up and slamming him over and over. Lesnar was a monster, he looked unbelievable and the fact he had Paul Heyman with him, and was referred to on commentary as "the hottest free agent" made me think that this was a guy who would dominate
Kane
Introducing a brand new character into a feud with Undertaker, and having Taker look fearful of him was unheard of. Kane was huge, he looked scary, he had Undertaker's former manager with him and when he ripped the door off the Cell to get to the Deadman, it was awesome. Kane immediately looked like he belonged in the ring with a performer like Undertaker and I immediately saw him as a future World Champion after that debut.
Chris Jericho
Who interrupts The Rock when he is in full flow. Someone who WWE have major plans for, thats who! Y2J never got a real chance in WCW, but WWE realised what a talent they had and allowed him to go toe to toe with their biggest star, and Jericho held his own perfectly. He moved up a level in his WWE debut and you could see he was going to do great things in the 'E.