The month of May proved to be a good one for the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO). It posted a gain of 2.4% during the period, making a new nine-month high in the process.

Most investors would be happy with this as a monthly gain, so imagine how shareholders of the following companies must feel. These three shares were the top three performers on the index in May, providing returns in excess of 25%.

Australian Agricultural Company Ltd (ASX: AAC)

Shares of Australian Agricultural Company rose 26% in May after it reported better-than-expected full year results. The company’s shares jumped to a seven-year high after it delivered a record after tax profit of $67.8 million. Australian Agricultural Company’s beef sales were the major highlight, with volume increasing 96% year-on-year. This means it now accounts for 88% of the company’s total sales. Things are looking positive for the company, but the lack of dividend puts me off investing in it.

Aristocrat Leisure Limited (ASX: ALL)

Aristocrat Leisure shares climbed by 25% in May on the back of a fantastic half-year report. The poker machine manufacturer delivered revenue growth of 47.4% to just over $1 billion, and statutory net profit after tax growth of 104% to $159 million. Its CEO Jamie Odell has stated its plan to target small acquisitions in the digital sector that will be accretive to earnings. This is part of its strategy of being a growth share, focused on growing larger in the medium term. I feel Aristocrat Leisure is an attractive long-term investment with a lot of growth ahead of it.

AWE Limited (ASX: AWE)

AWE Limited was the best performer on the index in May, climbing an astonishing 29%. This was the result of an announcement to the market that it had received an unsolicited, conditional proposal from Lone Star Japan Acquisitions to acquire the company for 80 cents cash per share. The junior energy producer’s board met and decided to reject the proposal, calling it “opportunistic”. Since then the share price has climbed up to 87.5 cents, with many investors speculating that Lone Star Japan will be back with a greater offer.

Foolish takeaway

Of the three companies I’ve discussed today, I believe Aristocrat Leisure is the most attractive investment. It has grown its earnings by an average of 24% per year for the last five years and I feel it is positioning itself well to be able to achieve strong levels of growth over the next five years, too. At 22x estimated FY 2016 earnings I think the shares are priced fairly for an investment with a long-term view.

If you feel like you've missed out on these gains, then don't worry. These exciting shares could provide your portfolio with a lift in the future. The great dividends they pay and their potential share price gains make them worth considering as investments today, in my opinion.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.