I'm going to start with my most played game and work backwards from there. So... K for Kerbal Space Program.
Maybe not the best choice in terms of number of games but I'd still get stuff like
- Knights of the Old Republic
- King's Quest
- Killer Instinct
- Katamari
- Kingdom Hearts
- Killzone
- Kirby
Probably others, but obviously not as extensive as a letter like S or T. Still I would probably opt to pick the letter with my favorite game of all time in there than one that just gives more options.