Hamilton moves to Mercedes following six years with McLaren, the first of which he teamed up with Alonso.
"I do not know who will be my main opponent for this season, he who has the best team, with the best year in terms of preparation, luck, testing, all of these factors, will help a driver or team to come on top of others," Alonso told Autosport in Italy.
"But as for strongest opponent, who is strongest driver? My answer is Hamilton. It was true last year, it was true this year."
"It is a personal opinion, not political, not to make people think something. Who is the strongest opponent, the strongest driver on grid? Who is the one you have to keep an eye on? It is Hamilton - and it will still be Hamilton next year."
"He is a super good driver because he won every year with any car: he won in 2007 and 2008. In 2009 they started around two seconds off the pace with the McLaren, and Hamilton was able to win races – and it was the same in 2010. Nico [Rosberg] we saw winning in China with Mercedes. So with Hamilton he will be able to win more than one race."
Alonso added that this did not mean a lack of respect for triple world champion Sebastian Vettel, who he believes "deserved these three championships".
Alonso believes McLaren, who won the final two races of 2012, will once again be strong.
"McLaren is always a strong contender. It is a team with experience and a history of good cars and continuous success. They will be strong again."
The Spaniard is nevertheless confident that he will start 2013 in better shape after Ferrari began 2012 on the back foot.
"I am more confident, no doubt."
"The rules are continuing more or less the same as last year in terms of aerodynamics. In the 2012 winter we changed completely the exhaust position compared to 2011, and it was a little bit of a guess what we found on the car."
"We can be faster or slower [than rival teams] but not 1.5 or two seconds off [of] the pace, because it was a record what we had last year. It is maybe impossible to be worse than last year, so I am confident."