Virgil Van Dijk
No matter who you support, you would have had to do incredibly well not to have kept up with the 'Van Dijk to Liverpool' saga that droned on and on from the summer right up until he signed for Liverpool a few weeks back.
The Dutchman's impressive performances on the south coast for Southampton were eventually rewarded with a switch to Jurgen Klopp's side over the new year, for a hefty sum of £75 million - making him the world's most expensive defender ever.
However, things could have been completely different for the 26-year-old if he had preferred the North East to Southampton back in 2015. Van Dijk appeared to be a very promising young player as he spent two years in Scotland with Celtic before making the move to the Premier League.
It was Dick Advocaat who had expressed his interest in the centre-back, but the race for his signature proved too difficult for us and Southampton later signed him up for around £13m. The fee may have seemed a lot to us at the time, but imagine selling him for £75m two and a half years later....
Benjamin Mendy's potential move to the Stadium of Light couldn't have been any closer to being finalised before losing out on the player.
The Frenchman had met with club officials in Sunderland after we had agreed a fee of €6 million, and was later pictured in front of the stadium posing with a fan. When you see a photo spreading across social media of a player right outside the ground then he has to be signing, right?
Unfortunately, Marseille matched our offer later that day and the prospect of playing Champions League football in France was understandably hard to turn down for the 18-year-old.
After a few years with Marseille, Mendy moved to Monaco, where a stellar first season saw him gifted the opportunity to move to Manchester City.
The French side made almost a £50m profit on the full back, who now - despite suffering from a long term injury - is set to win his first Premier League medal come the end of the season.
Sunderland's failure to sign Marcos Alonso on a permanent deal was simply just because his £5 million price-tag was judged to be a bit too much.
Despite a very impressive but short loan spell under Gus Poyet, once he returned to parent club Fiorentina that was it - our interest in the player ended.
Once back in Italy Alonso became a first team regular for Fiorentina, and his performances hadn't gone unnoticed by Chelsea boss Antonio Conte.
Only two years after we had refused to spend £5 million on the defender, Chelsea had snapped him up for around £25 million and the Spaniard became a key part of the Blues' title win last season.
It is painful to see yet another full back added to my list, considering how badly we have struggled in this position over the years.
Danny Rose was yet to become one of the best full backs in the league when he arrived here on loan back in 2012. The 22-year-old had a brilliant season under Martin O'Neill - providing us with both attacking and defensive options up that left flank.
The club very much helped Rose to blossom and become the player he has been at Spurs, however if we had pursued our interest in signing the defender then it could have been us that were able to cheer him on week in week out.
Granted, the England international wouldn't have stayed with us for too long, but we would have been guaranteed to see serious profit on his sale.
Yann M'Vila has to be the most frustrating possible transfer of the lot - as I'm sure many fans will agree.
He was absolutely outstanding during his loan spell with us and had made his desire to stay in the North East very clear. He had grown to love the club and everyone loved him.
The Frenchman looked like he was set to sign for us permanently, but it was a heartbreaking Instagram message posted by M'Vila that had fans raging.
The midfielder posted a message on the social media site, stating:
I called Sunderland but they are not picking up the phone, why? My heart has broken.
M'Vila would always be welcome with open arms back to the club, but unfortunately that ship has long-since sailed, and he signed with Ligue 1 side St Etienne last week.
Toby Alderweireld has easily become one of the Premier League's best defenders since joining Spurs for around £11 million in 2015.
The defender formed an impressive partnership with Belgian teammate Jan Vertonghen and the pair have looked very solid in what has been a very successful couple of seasons for Spurs.
However, prior to his switch to London, then-Sunderland manager Gus Poyet was looking to bring Alderweireld to the Stadium of Light on loan, but it wasn't much of a surprise when the Atletico Madrid man declined the offer due to our inability to offer him European football.
Possibly the weirdest of the lot - if all the stars had aligned, Swedish legend Zlatan Ibrahimovic could have been playing in red and white stripes alongside Kevin Phillips and Niall Quinn.
In fact, Peter Reid admitted he actually tried to sign the striker twice - once when he was at Malmo, and then at Ajax. The club had bid £6 million for the future star but Ajax had wanted more - which seems a very reasonable demand now.
Ibrahimovic later made the switch to Juventus for €16 million, eventually making his way to Barcelona for a reported fee of £59 million in 2009.
£6 million doesn't really seem that much now, does it?