GoodHousekeeping's Advice on Choosing Vegetables

Vegetables

Artichokes (spring): Compact, plump, heavy, with thick, green, tightly closed leaves. Avoid if leaves are dry, spreading, or hard-tipped.
Asparagus (spring to early summer): Straight stalks with closed, compact tips and full green color, except for white ends. Avoid if shriveled or have spreading tips.
Avocados (all year): Shiny green or mottled purplish-black (depending upon variety); yield to gentle pressure. Ripen in a paper bag at room temperature.
Beans, green and wax (all year): Firm, crisp, bright color without blemishes.
Beans, lima and fava (late summer and early autumn): Firm, crisp, smooth skins without blemishes.
Beets (all year, but best from summer to late autumn): Firm, small to medium-size, bright colored, smooth skins; preferable with fresh green tops attached. Avoid if bruised, soft, or overly large.
Broccoli (all year): Dark green, firmly clustered buds on firm, but not thick, stalks.
Broccoli rabe (all year): Perky leaves, with thin green stalks and small bud clusters. Avoid stalks with yellowish leaves.
Brussels sprouts (autumn through early spring): Firm, tightly wrapped green heads free of black spots.
Cabbage (all year): Firm, heavy for its size, with brightly colored (green or red) outer leaves and no black blemishes.
Carrots (all year): Firm, straight, with bright orange color, preferably with fresh green leaves attached. Avoid if limp or cracked.
Cauliflower (all year): Firm heads with tightly packed creamy white clusters and fresh-looking green leaves. Avoid those with blemishes or black spots.
Celery (all year): Crisp, pale green stalks with fresh-looking leaves. Avoid stringy, bruised, or limp stalks.
Chayote (all year, but best in summer): Small, firm, unblemished.
Corn (late spring through summer): Medium-size ears with plump, milky kernels, smooth green husks, and soft silk ends.
Cucumbers (all year, but best in summer): Medium to small, with bright green color. Avoid any with soft ends, blemishes, or wax coatings.
Eggplant (all year, but best in summer): Firm, glossy purple or white with fresh green cap, heavy for its size. Avoid if soft, wrinkled, or very thick-skinned.
Endive (all year, but best autumn through spring): Small, compact, snowy white leaves edged in pale green. Avoid wilted leaves.
Fennel (autumn to early spring): Firm, unblemished white bulbs with fresh looking feathery fronds attached.
Garlic (all year): Firm, unblemished heads with tight, compact cloves. Papery skin should be soft, not brittle.
Kohlrabi (spring through late summer): Small, young bulbs with fresh-looking stems and leaves.
Leeks (all year, but best in autumn): Firm, unblemished white base with fresh-looking green leaves.
Lettuce, greens (all year): Crisp, unblemished leaves; color depends upon variety. Avoid brown edges.
Mushrooms (all year, but peak for wild mushrooms like oyster or shiitake depends on variety): Firm, plump, unblemished with tightly closed caps and fresh-looking stems. Select carefully, avoiding mold. Best bet: Buy exotic mushrooms from a good gourmet produce store.
Okra (spring through early autumn): Young, firm, tender green pods.
Onions (all year, but certain varieties best in late spring or early summer): Clean, dry, firm with papery husks, and no sprouts or soft spots.
Parsnips (all year, but best in late summer): Firm, smooth, small to medium-size. Avoid overly large roots (a sign of age) and any with gray, soft spots.
Peas, green (spring through early summer): Firm, bright or light green, with well-filled pods. Avoid swollen, wrinkled, or immature dark green pods.
Peppers (all year): Firm, shiny, thick-fleshed with bright color, green, red, orange, or yellow (depending upon variety). Avoid blemishes, soft spots, or darkened stem ends.
Potatoes (all year): Firm, smooth skinned, well shaped, with no sprouts or blemishes.
Radishes (all year): Firm, smooth, bright color, red or white, with fresh-looking leaves. Avoid blemishes or black spots.
Rutabaga (autumn and winter): Large, heavy with no soft spots. Usually coated with a thick protective wax. Also known as yellow turnip.
Spinach (all year): Bright green, fresh, tender leaves with no yellowing or wilted ends.
Squash, summer or soft-skinned, such as zucchini, yellow, straight neck, patty pan (all year, but best in summer): Smooth, bright skin, bright color, green or yellow (depending on variety), heavy for its size.
Squash, winter or hard-skinned, such as acorn, butternut (all year, but best in autumn): Unblemished, rich color, green, white, yellow, or gold (depending on variety), heavy for its size, with hard skin and stem end intact.
Sweet potatoes (all year, but best in late autumn): Firm, uniform shape with even color and no blemishes. Avoid very large ones (sign of age).
Tomatoes (all year): Firm, plump with unblemished skin; color and size depends on variety.
Turnips (all year, but best in autumn and winter): Firm, unblemished, heavy for their size with fresh-looking tops.

source;Goodhousekeeping