Eternal Convention. This is the one con I attend every year at Whitsun... simply unmissable. The Eternal Con is the successor to the Tentacles convention, also held every year at Whitsun, and also in beautiful Burg Stahleck, a former castle that saw action during the Thirty Years' War, and which is now a youth hostel.
The Eternal Con has kept much of the focus of Tentacles: role-playing games tracing their descent from RuneQuest, such as the Call of Cthulhu and other Basic Role-Playing System-based games, and HeroQuest; and freeforms. The main difference between Tentacles and the Eternal Con is less emphasis from the latter on inviting 'big names' from the rpg industry, even though Jeff Richard from Moon Design was present this year, and Loz and Pete from the Design Mechanism were available through a G+ hangout — more on Jeff's, and Loz and Pete's panels later.
Yet another characteristic of the Eternal Con is its family atmosphere, and its friendly ambience, possibly due to the fact that many attendees have attended for years and know each other well.
I'll also mention the food, of excellent quality for a mere youth hostel, the incredible view over the Hunsrück and the Rhine (just look at the picture above!), and the glorious weather.