Most of the bosses are intense for different reasons…
Alien Queen/Snagglebeast: Depending on how you play the game, either of these will be the first you face, so it's kind of intense as it's the hardest thing you've done so far in the game…
Ultimate Supreme Executive Chairman Drek: There's that countdown timer thing he keeps activating, and each time he does so, the time you have to deactivate it is slightly shorter, so by the end of the battle, the amount of time you have to deactivate it means there's barely any time left by the time you've got there.
Tentacle dude on Planet Oozla: both parts of this boss (the main one and the bonus one) were pretty difficult, especially on the first playthrough, meaning this is a boss that I usually dread reaching, especially if I'm starting a new game…
The "thief": The first part of the battle against this character happens on a fast-moviing convoy of trucks through a snowy valley… it's pretty intense!
Thugs-4-Less Leader: He has a friggin' giant mech thingy!!!
Protopet: Recently, I've taken to playing through the games using only the starting armour; it makes it more interesting than buying the stronger armours, but trying to do this battle with the original Megacorp armour is really difficult, even if you've upgraded your Nanotech as much as you can, you can't afford to take more than two hits in the entire battle…..
B2 Brawler: The guy's like twenty times Ratchet's height!
Momma Tyrranoid: This one added to the drama by having those slow motion things where it chases you down the corridor in a first person viewpoint…
Gunship on planet Daxx: this one is intense in as much as that it just surprises you by just coming out of nowhere! There's no real reason or identity to this boss, and it's never mentioned again - it feels intense simply because it seems so… out of place
Courtney Gears: I felt that the arena design and soundtrack for this boss made it feel quite intense…
Giant Klunk: Like how the thief's battle took place on a fast moving convoy, this one takes place on a fast moving Sky-Train!!!
Doctor N.: He takes a lot of damage, and you'll most likely require the PDA and a lot of bolts to make it through. Therefore the battle is intense because you're constantly worrying whether you'll even have the resources to make it through the battle…
Ace Hardlight: They bigged this character up a lot to sound like he was a real combat pro, so when you finally face him, it feels quite nerve-wracking, as the character has such a reputation
Gleeman Vox/DreadZone power core: What really makes this battle intense is not the battle itself but the tasks that precede it, they add a lot of pressure given the consequences if you fail, and the boss comes in before you're fully able to recover from the stress it provides…
Luna: This one adds the extra pressure of having you occasionally have to dodge falling rocks as well as fight the boss (not to mention the chase before it), meaning that by the time you get to the end of the battle, you're fairly unlikely to have much health left.
Otto Destruct: Like with Neffy, this guy takes a lot of hits, and there's no way you can take him down without the PDA and a lot of bolts…
Klunk: This one is tough because he keeps changing into different battlesuits, all of which have different attacks (some of which I'm convinced aren't even dodgeable!), and parts of the battle utilise incredibly fast-paced Quick-Time-Events… (I couldn't even beat this one in challenge mode!)
Emperor Percival Tachyon: Again, this one has an awesome soundtrack, and, again again, if you're using the default armour, he can beat you in one hit! For that matter, even if you're using the Cragmite armour, which is, of course, the best armour you can get in the first playthrough, he can still kill you in about four hits!!!
Darkwater/Slag: This one was exciting because, not only does it take place across the whole level map, but also has two segment types, where he'll be invulnerable if he's going ghost.
VX-99: "Stall it? It's the size of a building!"
Cassiopeia: The very fate of Clank rests on this battle… also, like with the thief and Giant Klunk it's on a small surface, really high up.
Dr (Agai)N.: The parts where you were being chased down by the Hypernova Defense Laser were pretty horrifying! Plus there was the part of the Flying Saucer, like how with The Thief, Giant Klunk and Cassiopeia… yeah, you get the idea…
General Azimuth: Timey-Wimey-Explodey-Wodey….
Commander Spog: Thief, Klunk, Cassie, Dr. N…
Loki Master: This guy takes forever to beat, so it's a huge, satisfying relief when he finally goes down!
Stuart Zurgo: Like with Drek, there's a time-pressure thing
Neftin Prog: I felt that the setting, music, dialogue, effects and overall atmosphere for this battle were designed to perfection, it really feels quite epic! Plus I quite like how the Thugs ride in on trains which all come at you from random directions…
Mister Eye: The last segment where he keeps flipping you upside down was a good idea, and it's quite horrifying each time he does it simply because it makes you feel like you're going to fall! Also, trying to do this in Challenge mode that's also Legend mode is so insanely difficult! Especially given that one of his attacks can instantly destroy your Zurkons, effectively rendering them useless for the battle… And, like with Tachyon, even if you have the best armour, it still only takes him about four hits to finish you off……. oh, and let's not forget, Thief, Klunk, Cassie, DrN., Spog….