I have read about all of Twain, and most of it is very good ....
For novels, try Pudd'nhead Wilson, then A Connecticut Yankee. the latter is his best after Huckleberry. Wilson is a good read also, but A Conecticut Yankee is superlative. His young adult book, the Prince and the Pauper, about the exchange in identities between a young, rascally London pauper and the equally young Edward VI, son of Henry VIII, is a good read no matter what your age.
His biographical work is exemplified by Life on the Mississippi, which covers Twain's training to become a pre-Civil War river-boat pilot. Roughing It is about Twain's life in the silver/gold (?) fields of Nevada. This is where Twain wrote his great short story, The Notorious Jumping Frog of Calaveras County. On the submission of this story his where Twain invented his nom de plume of Mark Twain.
His two travel books: The Innocents Abroad and A Tramp Abroad show Twain's development as a humorist. While Innocents Abroad was written early in his career, it is worth reading. A Tramp Abroad demonstrates Twain's acquired skill using irony and hyperbole. His suggestion of climbing the Eiger in evening clothes is hilarious. When he wrote this, I don't think US evening clothes had been termed a Tuxedo as yet.
Twain was like Hemingway in that his short stories are some of his best work. These fill a complete volume by themselves, and should not be neglected. The Notorious Jumping Frog and The Man Who Corrupted Hadleyburg are among his best. As far as wasting money on Twain, it can't be better spent. I just spent the better part of a thousand dollars rebinding Huck Finn, Tom Sawyer, and Life on the Mississippi, all illustrated by the great Missouri artist Thomas Hart Benton. But Twain is worth it.
Good luck!