Hands up everyone who’s put off thinking about this year’s new £15,240 ISA allowance. You’re in good company. It happens to most of us after 5 April, when last year’s ISA panic has subsided. But it pays to invest as early as possible, and what better than three shares that should provide great returns for decades?

I’ll start with  Guinness owner Diageo (LSE: DGE), which has suffered a little due to a few tough years in emerging markets where its exposure is high. Although famous brands like Johnnie Walker are well-known here, they’re often much bigger in markets like China — and there are many brands in the stable never even seen on these shores. The result is that earnings have fallen a little over the past two years. And the share price fallen 3% in the past three years to 1,949p.But its five-year gain of 60% looks a lot nicer and has easily trumped the FTSE 100.

Longer term, there are billions of people in emerging economies who’ll want to satisfy their alcoholic desires in the coming decades. Diageo has so many of the best brands and is certain to pick up a lot of the trade. Forecasts suggest a bottoming-out this year and a return to growth in 2017, with City’s analysts rating Diageo a buy.

Diageo’s P/E is high at around 21, but if it has the long-term future I expect, then it could turn out to be an ISA bargain.

Pills and potions

GlaxoSmithKline (LSE: GSK) is a share price that has been struggling with a 9% fall over three years, to 1,497p, and just a modest 21% gain over five years. Although with dividends, those fives years would still have wiped the floor with a cash ISA! The stagnation is due to four years of accelerating earnings falls due to the loss of patent protection for key drugs and increasing generic competition.

But after several years of big investment in its drugs pipeline, Glaxo should return to earnings growth this year. And once quarterly profits rise, we could see the start of a share price rerating. Falling earnings have pushed the forward P/E up to 17.6 for this year, dropping to 16.8 on 2017 forecasts. But that looks cheap to me if the expected recovery is just around the corner.

And there’s a bonus in the dividends, which Glaxo has kept going. Forecast yields of more than 5% would be barely covered by earnings, but that shows a confidence in future earnings growth that’s not misplaced.

Oil, really?

Anyone investing with a decades-long horizon should have a big oil company in there, and they don’t come much better than Royal Dutch Shell (LSE: RDSB). The falling oil price has caused a 20% fall in Shell’s share price over five years, but dividend yields of over 5% annually have left investors in profit. We have 7% yields forecast for this year and next, after Shell maintained its 2015 payout with no words of warning. It’s not certain to keep paying, but I reckon Shell will want to keep up with BP‘s commitment to see out the crisis without cutting the cash.

Whether the forecast return to growth comes off in 2017 depends on where oil prices go over the next year or so, but I have no interest in the timing. Oil will recover longer term, and that’s all that matters for a great ISA candidate like Shell. There’s another strong buy recommendation from the City and I just can’t disagree.

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Alan Oscroft has no position in any shares mentioned. The Motley Fool UK has recommended BP, Diageo, GlaxoSmithKline, and Royal Dutch Shell. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.