Ask 10 different people, you will likely get 10 different answers as to what constitutes a /good/ mod. It really depends on what interests you. Yjere are mods that range from complete and total cheating to massively unfair hard to play. For Instance, one i use is a Diamond Meter... sounds like a cheat at first, but to create it you need a block of diamonds and 2 more diamonds, and it doesn't point you STRAIGT to diamonds. Others allow you to build a fully automatic mining machine (Buildcraft)... but it will be some time before you can afford the resources to make and power it. Others are just aesthetic in that they let you make colored beds, stairs from other materials, show recipes that would normally have to be looked up in mod threads or the wiki.
I Suggest The Dragon Mount Mod, You Can Spawn A Dragon Egg, When You Right Click It, It Will Start To Grow And Very Soon A Baby Dragon Will Grow Out Of It. When It Is Fully Grown Into A Grown Ender Dragon, You Can Tame It With Raw Fish Then It Will Follow You (A Bit Like Wolves) If You Click It With A Bone, It Will Sit. You Can Put A Mount On It Then You Can Fly It. You Normal Fly It With The Arrows But If You Hold A Rod With A Carrot Then You Can Control It With The Mouse. You Can Heal It With Raw Fish And Man Other Food If Your Dragon Get's Hurt. You Can Also Re-Name You Dragon. I Personally Love This Mod, Click Here: http://www.minecraftforum.net/topic/827863-15x-dragon-mounts-0992-wip/ Further Info Would Be On The Forum Page By Clicking The Link. Thanks!