The more frequently a company makes it into the Top 20 of this annual survey of leading listed companies, the less likely it should be that the feat can be repeated.

That's because the methodology draws both on historical performance and likely prospects for the investor in the future. It stands to reason that if a company does extremely well for several years running, it will probably have become fully priced. No rate of growth is sustainable forever, though it may have seemed otherwise when the expansion of China's economy in the early to mid-2000s was running at 10%/year, or with the 20% year-on-year rise in SA house prices in about the same period. The classic blue-chip share is one that you can count on to grow in all market conditions - but even the blue chips find it difficult to keep growing strongly, while still attracting investors who are looking for value.

That's why the achievement of MTN is so remarkable. It is the only company that has appeared in the Top 20 in every one of the seven years that the present format has been used. MTN just keeps doing well, each time off a higher base. In a sense, it makes things ever more challenging for itself - yet it keeps exceeding expectations.

MTN's growth is due partly to the sector it is in, where the phenomenal expansion of cellphone usage has moved beyond saturation, as voice is supplemented by all the other functions that convergence brings. MTN's early vigorous expansion into the rest of Africa, as well as bold (some said reckless) investment in the Middle East, has given it prime position and critical mass for further growth.

As downward pressure on prices increases, the biggest cellular players will be able to take advantage of economies of scale. There will be more consolidation, which should also be good for MTN shareholders.

The other world-class SA player in this market, Vodacom, was hamstrung geographically for years in Africa because of the restrictions imposed by major shareholder Vodafone. Now that Vodafone controls Vodacom, that falls away - but in some ways MTN is already beyond the horizon.

Vodacom is also undergoing leadership transition, with iconic founder Alan Knott-Craig replaced as CEO by Pieter Uys, at a time when the demands of being listed and dealing with a new owner must inevitably distract management's attention.

MTN, on the other hand, still has Phuthuma Nhleko in the hot seat and there can be few executives who have delivered as much shareholder value as he has. His confidence and ability to take risks have marked him out. In 2006 this publication noted how he was prepared to pay US$5,5bn for Lebanon-based Investcom, which represented a 27% premium to that company's share price at the time. Nhleko explained that "this was quite reasonable in the context of telecom operators nowadays".

The first criterion in the historic quantitative analysis of the top listed companies is that it should have a market capitalisation of at least R1bn, so that investors can be confident that an operation is sustainable and has critical mass. The second criterion is a consistent track record over the previous five years.

About one-third of the overall score (quantitative and qualitative) is derived from a five-year assessment of internal rate of return (IRR) and compound growth in EPS. In this way, factors that are within management control, as well as those that are more market-derived are incorporated. IRR measures a company on its returns to shareholders - a combination of dividend flows and capital appreciation in the share price, also known as total shareholder return (TSR). The balance of the 40% of the total score is derived from return on equity in the latest year under review, which means that recent success is also rewarded and a company is not entirely a prisoner of its history.

The numbers, as always, are crunched by the Bureau for Financial Analysis, which aims to eliminate variations in the ways that companies report their financial performance by standardising treatment of the figures. The results don't always look the same as the financials published by the companies themselves in year-end presentations or the annual report, nor do all the year-ends coincide. But the calculation is consistent from year to year, so at least apples are being compared with apples.

Because the aim is to use the past as a guide to, rather than a determinant of, the future, 60% of the score is based on a largely qualitative assessment of how "investable" a company is.

Factors included here are how the company is managed (and is likely to continue to be managed); its corporate governance procedures and culture; its black empowerment status (though increasingly this is taken as a given with the best companies); the quality of communication with shareholders and stakeholders; the prospects for growth in the sector or sectors in which the company operates; contextual issues such as regulatory uncertainties and tax regimes; and, crucially, whether the share is reasonably liquid and offers value that the herd may have missed.

In terms of market cap, MTN (R202bn) and BHP Billiton (R396bn) are by far the biggest companies in the Top 20. Of the rest, only six have a market cap of more than R10bn. This is not surprising - the bigger you are, the harder it is to grow and the less likely you are to be agile.

Two of those six are companies linked to the resources boom - Assore (R13bn) and ArcelorMittal (R39bn), and therefore they can be expected to show less impressive growth in the next survey. But two are retailers, Massmart (R17bn) and Shoprite (R28bn), both of whom have probably surprised most observers with their resilience.

Early results for the next review year indicate that both will ride out the recession with confidence, probably with increased market share. Another retail-linked share is Tiger Brands (R24bn). The remaining stock with a market cap of more than R10bn is Naspers, which has used its near-monopoly of the TV sports and entertainment market to generate cash for aggressive foreign acquisition.

The obvious absentees, of course, are the property companies. None appeared in the Top 20 last year, but in 2007 the property loan stock company Resilient appeared as high as fourth.

The leading giants by market are so big that changes in the order at the top of the table tend to be glacial. Five years ago, the top five by turnover were BHP Billiton (R140bn in 2004), Anglo American (R124bn), Old Mutual (R70bn), SABMiller (R65bn) and Sasol (R64bn). This year only Old Mutual has dropped out of the top and is replaced by Sanlam. SABMiller, having nearly tripled its turnover, moves into second place ahead of Anglo American.

Whereas BHP Billiton has gone from R278bn to R470bn in a year, Anglo has dropped from R232bn to R174bn. This provokes interesting questions about whether, of these two leading resources companies, Anglo now offers more value to investors - and of course various other measures are provided to aid this assessment.

Moving up the ranks, of course, is MTN, which has gone from 23rd on a turnover of R19bn in 2004 to ninth on R73bn. But MTN has the fourth-highest market cap on R202bn, ahead of companies with much bigger turnovers such as Sasol, Sanlam, Bidvest and Standard Bank.