The short answer
Salad leaves, herbs, tomatoes, courgettes, runner beans, chard and soft fruit. All of them are either expensive to buy, poor in the shops, or vastly better fresh.
- Salad leaves: cut-and-come-again mixes crop for weeks from one sowing, and bagged salad is expensive and short-lived.
- Herbs: the single best value, particularly the ones sold in tiny expensive packets.
- Tomatoes: genuinely different from shop-bought, and that is the whole point.
- Courgettes: one or two plants feed a family, and they need almost no skill.
- Runner and French beans: heavy croppers in a small vertical space.
- Soft fruit: raspberries, blackcurrants and strawberries, all expensive to buy.
- Not worth it in a small space: maincrop potatoes, onions, carrots and sweetcorn, all cheap to buy and space-hungry.
What will ruin it
Do not plant more courgettes than you need. Two plants is generous for most households, and by August anybody with four is leaving them on neighbours' doorsteps.
Why it works
The economics of a small garden are about value per square metre and about the things industrial supply chains do badly. Tomatoes and salad are picked unripe and chilled, which is precisely what ruins them.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.