mine would be -
1.parrot-ugly and unnatural
2.flowerhorn-manmade(although like those that look like trimacs)
3.discus-too much of a bother
4.pacus-too big,can be ugly
5.giant gouramis-hideous lookin
6.many of the large catfishes-shouldnt really be classed as aquarium fish ,imo. 

A long list from me starting with the horrid man made varieties and the blatantly unsuitable:

1.  Coloured parrot cichlids
2.  Parrot Cichlids
3.  Any variety of 'balloon' fish (molly, ram etc)
4.  Flowerhorns (although I recognise the non-balloon varieties are beautiful, I'd never keep one.  Give me a good coloured Red Devil (Citrinellum) or Festae anyday over one of these. 
5.  Celestial goldfish
6.  Pangassius catfish; has no place in any tank unless you have a 6-8 footer as an absolute min.  They are large food fish in Thailand growing to 24 inches plus. 
7.  Cleaner wrasse.  Don't even go there.  They don't eat whitespot whatever anyone might tell you.  Unless you have a huge tank with at least 50 decent (tang size) fish in there, they aren't going to survive no matter what your LFS tells you! 
8.  Most marine butterflyfish that are obligate polyp eaters.  Continuing to import these varieties and sell them plays directly into the hands of the the 'environmental lobby' who want to regulate orur hobby out of existence. 
9.  Discus that aren't discus shaped any more (too much indiscrimiate in-breeding)
10.  Veiltail angels (unless in seriously good condition in which case they are wonderful, most just look a bit scraggly). 
11.  Long fin varieties of ruby barbs, rams, danios and the ultimate abominations Long-Finned Koi and Oscars.  Makes me cringe just looking at them. 
12.  inbred malawi cichlids of no determinate variety. 
13.  Poor quality fancy goldfish that look like no known variety. 

I'm not alone .. there are many who don't like keeping discus in their tank.  We keep tank to relax ourselves, not to be the slave of the fishes.  They need hell lot of care and dedication, which is very hard to give. 
Their looks are awesome but caring for them takes high time. 

here goes my list -
1.goldfishes - i guess they are the most attractive,but honestly speaking i hate them.messy buggas. 
2.all types of livebearer's - mollies,platies,swordtails. 
3.channa's (sorry to offend fellow iih'ians) - no specific reason.i just hate them.most of the channas i have seen are lazy buggas. 
4.red tail shark/rainbow shark - they are so aggressive towards their own kind.you can only keep one in a tank. 
5.ordinary pleco's - good for cleaning a tank,but when they grow big,they become messy. 
6.flowerhorns - i hate them for their bulging head.looks like a tumor
7.tiger sharks - not attractive at all. 

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