These are a selection of games that have a special place in my heart, whether they were just hard in general, had some really difficult achievements or just took some ridiculous grinding. Sorted by playtime.

The Binding of Isaac - 230 hours
Dark Souls 2: SotFS - 205 hours
Dark Souls - 154 hours
Dark Souls 3 - 117 hours
La-Mulana - 104 hours
Crash Bandicoot 4 - It's About Time - 96 hours
Hammerwatch - 80 hours
Descenders - 62 hours
Action Henk - 54 hours
I Am Fish - 47 hours
Tricky Towers - 44 hours
Surgeon Simulator - 37 hours
Bloody Trapland - 34 hours
Pid - 33 hours
Electronic Super Joy - 29 hours

Mortal Kombat Komplete Edition - 453 hours
Adventure Capitalist - 386 hours
A Valley Without Wind - 158 hours
Final Fantasy V - 155 hours
Nation Red - 112 hours
Chivalry: Medieval Warfare - 102 hours
Speedrunners - 85 hours
Disney Epic Mickey 2 - 57 hours
Hand Simulator - 42 hours
Q.U.B.E. - 38 hours
Retrowave - 29 hours

Life Goes On - 38 hours
SteamWorld Dig 2 - 21 hours
DLC Quest - 16 hours
Braid - 12 hours
SteamWorld Dig - 12 hours
Pile Up - 10 hours
Another Perspective - 7 hours