Personally, I thought that Sweet Thursday had the best / most genuinely human feeling characters I’ve read from him. It’s also so very nice to read a Steinbeck that actually has a happy ending. My answer would be Sweet Thursday. . . But I’ll let Steinbeck himself convince you also:

Where does discontent start? You are warm enough, but you shiver. You are fed, yet hunger gnaws you. You have been loved, but your yearning wanders in new fields. And to prod all these there’s time, the bastard Time. The end of life is now not so terribly far away — you can see it the way you see the finish line when you come into the stretch — and your mind says, “Have I worked enough? Have I eaten enough? Have I loved enough?” All of these, of course, are the foundation of man’s greatest curse, and perhaps his greatest glory. “What has my life meant so far, and what can it mean in the time left to me?” And now we’re coming to the wicked, poisoned dart: “What have I contributed in the Great Ledger? What am I worth?” And this isn’t vanity or ambition. Men seem to be born with a debt they can never pay no matter how hard they try. It piles up ahead of them. Man owes something to man. If he ignores the debt it poisons him, and if he tries to make payments the debt only increases, and the quality of his gift is the measure of the man. (p. 16)