What style of defense do you find the most fun to watch?

Loma of course. He makes combining offence and defence and blending them together seamlessly look like pure poetry in motion, slipping into range to land his shots whilst making you miss with your shots and then slipping back out whilst again making you miss. He's easily the most skilled fighter in the game and it's not even close

Usyk of course

Duran

If we're talking about the American school James Toney

 

Loma love incoming

Let's just rewind back to Loma's insane amateur career and some equally crazy stats

This is his trophy cabinet

He won every single tournament he entered as an amateur in almost 400 fights sans one where he came second. Just think about that for a second

One solitary silver medal in an ocean of gold

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Imagine coming back with a gold medal in every single tournament you entered as a child, teen and man and only failing to do so once where you came back with silver which was your only loss in almost 400 fights, loss avenged x 2 and it's his only loss in over 420 fights amateur and pro combined against someone close to his size not between 2-4 divisions heavier and 1 of only 2 legit losses in all those fights

Another crazy stat

He conceded only 7 points when he won the gold medal at the 2009 World Amateur Championships despite the fact he broke his hand in the tournament. Just 7 freaking points in the whole tournament.
Most amateur greats concede that amount in single fights

'Even a broken hand couldn't stop Lomachenko as he won gold at the 2009 World Championships in Milan while compromised. In that tournament, he beat his five opponents by a 56 point margin, outscoring them 63–7.'

And again

'Lomachenko won gold at the 2008 Olympic Games in Beijing and avenged his 2007 World Championship defeat to rival and reigning world champion, Selimov, in the first round. He beat his five opponents by an astonishing 45 point margin, outscoring them 58–13, en route to his first gold medal at the senior level. He was subsequently named the outstanding boxer of the tournament and awarded the Val Barker Trophy. His dominant run was described by one historian as "probably the most impressive streak in Olympic boxing ever."

Additionally, he won gold again that year to also become continental champion at the European Championships in Liverpool, conceding just 4 points against his four opponents

I don't think there is any amateur in the history of the sport no matter great they were in the amateurs or pros who can boast such insane defensive stats as those and even more impressive Loma isn't a runner or an octopus