Labour's Carwyn Jones said his government would fight Wales' corner, while Conservative Andrew RT Davies promised a "brighter future".
Plaid Cymru's Leanne Wood pledged to share her vision across Wales in 2016, while the Lib Dem's Kirsty Williams said her party was "up for the fight".
UKIP is also expected to win its very first seats in the assembly in May.
Giving his New Year's message, the Welsh Labour leader highlighted the organ donation laws as a defining moment in 2015.
"For me, one of our greatest achievements of the year was Wales becoming the first country in the UK to introduce a new system for organ donation," said Mr Jones.
He said his government continued to invest "record amounts" in the Welsh NHS, while delivering improvements in education, and economic growth that "continues to outperform the UK as a whole".
He also wished the Wales football team well ahead of the Euro 2016 finals in France.
But with a fight on his hand to win outright in May's elections, the Labour leader added: "We'll never take our foot off the pedal, we'll always fight Wales' corner, striving for the best. The people of Wales deserve nothing less."
In his address, the Welsh Tory leader said 2016 gave Wales the chance for a "fresh start".
"After 17 years with Welsh Labour at the helm, Wales finds itself at a crossroads and there are big decisions ahead," said Mr Davies.
He said voters had a straight choice between "the change Wales needs for a brighter future, or sticking with Jeremy Corbyn's Labour Party".
Leanne Wood takes her party into the Welsh general election as leader for the first time, stating she will be travelling the "length and breadth of our country and making the case for Plaid Cymru and sharing my vision for Wales".
"On that election day, the path Wales chooses will be in your hands. Forward with Plaid or more of the same, from the same old politicians."
Liberal Democrat leader Ms Williams said Wales was "crying out for fresh ideas and a government that puts people first".
"In 2016 our mission will be to show people that we will help them reach their aspirations, that we have a record of delivery for our communities and that our values are needed in the assembly now more than ever," she said.
But while all four party leaders push their own messages home, they will also be keeping one eye on the challenge from UKIP, and what it may mean come 5 May.
Opinion polls have suggested UKIP could win several seats in the Senedd.
Their Welsh leader Nathan Gill told his party conference in the autumn that he was "extremely hopeful" of winning seven or eight assembly seats.
"I have long predicted Wales will be one of the strongest regions for UKIP in the United Kingdom," he said.
"Next May will see that come to fruition."

Phil Flanagan was ordered to pay nearly £50,000 damages to Tom Elliott following comments he made in a tweet.
Mr Flanagan said he was unaware his tweet was libellous.
Lawyers for AIG argued that he is ineligible for cover because he knew the comments were defamatory.
They also contended that Mr Flanagan is excluded as the social media post had nothing to do with constituency or parliamentary business.
But the Sinn Féin representative for Fermanagh and South Tyrone said he was unaware the tweet was libellous - originally believing he was merely posing a question after listening to a "one-sided" radio discussion on Northern Ireland's troubled past.
The judge in the libel case said that Mr Flanagan's tweet was grossly defamatory.
Following the two-day insurance hearing at the High Court in Belfast, the judge said he pledged to deliver his verdict as soon as possible.
A High Court judge has put a stay on any payout until Mr Flanagan resolved his legal action with the  insurers.
The assembly's four party leaders have delivered New Year messages with an eye on the 2016 elections.
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Judgement has been reserved in a Sinn Féin MLA's legal battle with an insurance firm over its refusal to to provide cover for a libel award against him.