I don't put a massive amount of stock in first impressions of a wrestler. Just like in real life, first impressions sometimes don't often give you the whole story on people.
For example, in TNA, they've had the Gut Check concept going for the better part of a year. Now the concept in and of itself is essentially built around first impressions. In my eyes, and in the eyes of many from what I've read & heard, the vast majority of those Gut Check competitors made lousy first impressions. It doesn't mean that they can't be or aren't good wrestlers, but they certainly didn't convey that impression on most people during their first TNA appearances.
Guys who didn't particularly floor me when I first saw them include guys like Triple H, John Cena, CM Punk, Cody Rhodes, Dolph Ziggler and others. I wasn't crazy about their characters or their mic work or whatever, but they didn't make a positive first impression with me. Look at them now: Triple H will probably be running WWE in the next couple of years and is has had a Hall of Fame career. John Cena has been the face of the company since the mid 2000s and will also be in the HOF someday. CM Punk is at the top of his game and, generally speaking, has probably been the most over guy in wrestling as a babyface or a heel since last summer. Cody Rhodes is a young guy with a promising future who has established himself as a rising star in WWE and same goes for Dolph Ziggler. So yeah, first impressions don't always tell the whole story, sometimes it doesn't tell the story at all.
As for guys that made big first impressions that did make good, the top of my list would probably be Sheamus. From the minute I saw him for the first time on Raw, I felt that the guy had the ability to be a major player in WWE. He had the look, he had the personality and he had the in-ring ability to stand out.
Another guy is Damien Sandow because of just how smoothly he spoke on the mic and how comfortably he fit into his current character. Sandow might wind up crashing & burning as he hasn't been around for very long but, all in all, strong first impressions and he's progressed nicely.
There've been lots of others as well but, sometimes, it takes longer to really make an impact. Look at Daniel Bryan, for instance. The first time I saw him, he showed that he had the stuff in the ring by wrestling against Chris Jericho on the first episode of NXT. I wasn't really sold on his promo abilities or anything until he was "eliminated" from the show and delivered a very passionate promo. I knew then that Bryan had the ability and the personality to get over with fans if WWE simply gave him the opportunity to really speak and develop a character. Now, he's one of the most over guys on the roster.
"What Do I Know Of Cultured Ways, The Gilt, The Craft And The Lie?
I, Who Was Born In A Naked Land And Bred In The Open Sky.
The Subtle Tongue, The Sophist Guile, They Fail When The Broadswords Sing.
Rush In And Die Dogs - I Was A Man Before I Was King."
Conan Of Cimmeria