Whilst Drogba's first two seasons were not memorable as he struggled to get a starting spot, by 2006, the Chelsea legend became an integral part of their Premier League success, including 29 goals in 32 games in 2008/09.
As mentioned, Arsenal had already won the Premier League prior to Henry's arrival, so Bergkamp's transfer was slightly more important. However, Henry is perhaps the greatest this league has ever seen and he has to be included.
Robin van Persie
No, not to Arsenal, to United. Indeed, Robin van Persie is an Arsenal legend despite the volatile transfer to their arch-rivals, but with 26 goals in his debut season at United, the Dutchman practically won the club's record-breaking 20th title by himself.
Whilst much of Leicester's Premier League-winning squad was with them in the Championship, Robert Huth was brought in for experience to help them stay up as they fought relegation in January 2015. Huth would sign officially a few months later and he would play 35 league games and form a sensational partnership with Wes Morgan that conceded just 36 goals all season and beat the 5000/1 odds to win the title.
Kompany might have been the first of City's golden era, but Aguero was the goals. The Argentine was far from the first marquee signing at the Etihad, but with 23 goals in 32 league appearances in his first season, two on his debut, and *that* last-minute goal that won the title in his debut campaign, arguably nobody has had a better first season in English football.
With Andy Cole leaving two years prior to Les Ferdinand joining Newcastle, the Magpies were seriously lacking goals. Ferdinand would sign in 1995 and score 25 goals in 37 appearances in his first season before Alan Shearer joined him a year later to form a formidable partnership. Newcastle would finish second for the two seasons Ferdinand was there and they infamously conceded a 10-point lead to United after leading the league from the start until March in 1996.
Liverpool needed a replacement after selling Fernando Torres to Chelsea in 2011 and with Luis Suarez, they didn't just replace him, but made an improvement at the same time. His first full season brought in just 11 goals, but with 23 the next season and 31 the campaign after, the Uruguayan became a Premier League legend. In fact, despite losing the league by two points on the final day, the ex-Liverpool man arguably had the best individual season of all-time with 31 goals in 33 games and if he hadn't had missed the other five through suspension, no doubt he'd have smashed the record for most in a season.
United were already a winning team when Cristiano Ronaldo joined the club, but with the rise of Chelsea and Arsenal's growth with players like Henry and Patrick Vieira, plus the departure of David Beckham, they needed a new star. Ronaldo didn't start to hit the ridiculous goalscoring numbers that has now become the norm until 2006/07, when he won his first league title with the club, followed by two more consecutively, but, don't get it twisted, he was an instant impact at Old Trafford.
Salah, after being sold by Chelsea, returned to the Premier League with 32 goals in his debut season at Liverpool, and nobody in Premier League history has ever matched this. This in itself is enough to have a shout on this list, but considering he also helped bring the Premier League title in his third season and a Champions League in his second season, it's fair to say that his impact is hard to beat at Anfield.