As TVF said the what's good on defense question is division dependent but also roster, play style and opponent dependent.
E.g. at the lower GP of div 4 and div 2 you may be playing with not enough well geared teams so you might need a really strong defensive squad that your opponent will struggle on to make up for some weaker team. At the higher GP end of div 3 and div 1 you might be able to set a very "flat" level of defense teams that just take protection banners away from your opponent and maybe if you are lucky drain them of certain specific counters before they full clear. Meanwhile if you set bossk lead BH vs someone with well geared troopers and a fast stark you are giving them 60 banners vs maybe giving someone without good troopers a slightly harder time.
My personal generalized thoughts though which I'm sure differ to a lot of players:
Gas; Defense because no protection regeneration and requires strong counters not yet well practiced
Sith Empire; Defense because requires strong counter and will cost banners. Also offense; undersized win with out malak vs almost anything and then use malak for a 64. Or mirror SE.
Padme; Opposite of SE as it's a possible counter but also beats basically everything else. Can be good on defense if you include 3P0 and also use Bugs (double up on traya counter)
JKR; Offense because regen. prot., defeats almost anything specifically GG.
GG; defense because high chance it costs opponent banners to beat and would otherwise cost you banners on offense vs most teams of similar power / investment
Clones (without GAS); defense under rex lead (harder to beat than shaakti lead) or offense under shaakti lead (counters most things including SE if JKA included)
Bugs; depends on other factors, e.g. can you make them use at least Traya and DR before coming across them. Otherwise they beat an awful lot for high banners on offense.
CLS; Offense (typically) as he counters a lot of teams usually undersized (specialty counter for higher end teams also). Can surprise people when used on defense though.
Traya; Offense as she counters a lot of teams specifically padme and bugs.
Nute/Dooku/Seps; Defense as still unfamiliar for some people. Many comps. e.g. Wat, Jango, Nest. DDK works if you need a BH team somewhere.
NS; At some point will have to be moved to offense as everyone puts them on defense and eventually people are going to learn how to beat them without screwing up. For the moment though enough people occasionally slip up or waste some banners on them so they are still worth defense. They cost you 5 banners on offense.
Carth; very often seen on defense, have a high banner counter prepared (e.g. malak solo, CLS undersize or FO). If used on defense do something to make it at least a tiny bit of a challenge (add L3, Wampa, Nest or Wat or g13 your canderous)
BH; try not to use bossk on defense as everyone has had over a year practicing against it always being on defense and jango lead is better if you get the speeds and turn order right. Can be good on offense especially if you practiced a bit with them against NS for example.
FO; with a few g13s and great mods can trip people up on defense. But they have prot. regen., can be used vs carth or for cleanup vs e.g. a lone malak. Well geared, modded and practiced they can be used for 60 against relic'd NS.
qira or clone wars chewi + scoundrels; tricky little defense filler team if you have the team comp. and gear for it.
JTR/Palp/Troopers/Ewoks; gearing and modding dependent offensive counters to a lot of filler lever teams. Can be occasionally reused on defense vs players with a big mod advantage as their TM gain mechanics can outweight that a little. Optimal on offense as and when needed though and they have many high banner counters.
Nest/wampa in general; dependent on if you often see teams you can solo (I haven't seen one for ages) otherwise I prefer use to reduce the number of counters available to beat filler teams on defense.
R1/Phoenix/GR Jedi/Tuskans/Lobot..... if you find a use for them more power to you.
Lastly all of that goes out the window as soon as you review the opponents roster and spot a weakness you can exploit or a strength you need to compensate for.