These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Crowning Moment Of Awesome - R-Type Final's final stage, and how you beat the final boss. Almost all the best tracks are played back-to-back in Stage F-C.
Crowning Moment Of Funny - Debatable considering the overall seriousness of the series, but the Bydo Database remark in Final about a ship with regenerative abilities (B-1C3 Amphibian III) being unpopular because of its tendency to drain all of their lifeforce and the followup deadpan "Dead pilots cannot be regenerated" is at least somewhat amusing, as is another ship's similarly feared cockpit...which has a test-tube shaped canopy, complete with volume markers.
The scientists insisting that the Platonic Romance is powered by love. The writers seemed to have quite a bit of fun with some of the ships.
Final's soundtrack had the "R-Type Medley Arrange", which was a compilation remix of several songs from the series. You're hard pressed to find another song that serves as the franchise's crowning music.
The music from the C64 port, especially the title theme.
Freud Was Right: Really, really right. Sperm is not an uncommon type of ammunition, one boss looks like a vagina being penetrated by giant worms, there's that... background from Final, and the final boss of Delta is a giant ovum.
Fridge Brilliance: It's not likely the Bydo are constraining themselves to attacking just the 23rd century and its immediate vicinity. Why not attack the far past, while you're at it? Oh, right, they probably have. Made from the vilest potentials of the human psyche? Meet the Legions of Hell...
Fridge Horror: Beyond the Bydo themselves, there is this question: exactly what kind of horrific enemy was future humanity fighting that they thought the Bydo were an acceptable weapon?
Bitter Chocolate answers this. There's an "Other Civilization" and they apparently manage to do something that make the Bydo do a 180. Let's just leave it at that.
Game Breaker - Final's B3-C2 Sexy Dynamite II. Let's get this straight: you've given me a really strong Force, no slouch of a Wave Cannon, and not noticed that the way the Force is constructed causes it to charge the Delta Weapon anything up to seventeen times faster than a Standard Force when separated? When you can hit the same boss with multiple Delta Weapons that most craft can only use once per stage, something is very badly wrong. Note that the Ragnarok II doesn't count, because they did that on purpose.
Then there's the R9-Leo IILeo2, which is stronger than the first R-9Leo because of an enhanced force module, bits and wave cannon, and it's Claw Laser+ is capable of killing large enemies and bosses in seconds.
THEN there's the Pow Armor line-up. They're those power-up transports that you shoot down throughout the game but are formidable player craft on their own. Their force modules are relatively powerful when attached and their wave cannons do strong damage, but detaching the force module unleashes a flurry of bullets that can kill enemies as quickly as the Leo2 and the Sexy Dynamite II.
The Strider in Final was pretty worthless, but in Tactics it becomes one of these due to its Balmung nuclear warheads, which do extraordinary damage to pretty much anything and everything. It can only carry one at a time, but that is fixed by simply having a POW Armor follow it everywhere, reloading it after each shot.
On the subject of Tactics, the Gains unit in the Bydo campaign is hands down the most broken unit in the game. Its wave cannon shot not only does a large amount of damage AND covers a large range of space, it also takes only ONE turn to charge up. And you can have a large number of these units. The only downsides to this are that your charge resets if something touches your unit (though this can be remedied by simply having another Gains nearby), and that it only fires forward, which is only a problem in 2 or 3 missions.
The original R-Type 3 version of the R-90 was so powerful even it's game of origin that they had to split it's abilities across 3 separate ships and even then 2 of those ship are still broken due to the way the Mega and Giga Wave cannons have absurd power (both can kill damn near anything at just 2 loops and the Giga wave cannon of the R-902 can blast even stronger than that).
Goddamned Bats - Cancers, a common enemy in all the games, have the ability to move and fire outside the normal eight directions and appear on the wrong side of the screen whenever the programmers hate you.
Nightmare Fuel - Final's worst ending. You never even realise what's wrong...
The Bydo Empire enjoy using this as a part of their total warfare against humanity on all fronts. They want humanity to feel fear and confusion as to what they are.
Not to mention the entire concept of the Bydo's "wave" form. Imagine that for a second. A wave, as quick and unobtrusive as a sound. And all you need is to get close for it to begin taking over.
R-Type 3 features a level which appears to be set in a gigantic alien's stomach, until you reach the end of the level and the boss is a series of orifices that deploy giant sperm at you. And then there's Cyst / Gomander, one of the most disgusting vagina monsters ever created.
The Final version of Dobkeratops features a phallic tongue that appears to deploy sperm. The vagina boss also makes a comeback.
Stage 5 of Delta. Fending off enemies from the original R-Type wouldn't be so bad if not for said enemies being dilapidated husks and the acid-trip landscape. And things getworse in the last two stages.
Bellmite, the fifth boss of the original R-Type. The core's covered by an outer shell that it fires at you piece by piece. The pieces are incredibly durable for their numbers, and Bellmite LOVES to trap you in a corner at times. At least in the first loop, the ring laser is your friend.
R-Type II has both Corvette and Rios. The former has very fast projectiles coming from everywhere, from the turret circle guarding the cores that aren't currently flying around blasting you, to the shrapnel from the bombs it fires. The latter has you go around a long obstacle course very quickly while he blasts you with drill missiles that make the narrow tunnels more narrower and lots of lasers out of the core you're supposed to hit. Oh, and if you're in the second loop and have a maxed out Force, you're screwed.
That One Level - Three words that will make any R-Type III veteran squirm: "Backwards Laser Maze". In the fourth level, there is a truly nasty maze of passages in which lasers or plasma or something periodically takes seemingly random paths through. It's very difficult to find your way... and then you have to do it again, backwards!
The fiery laser stuff is actually molten metal, which is why it moves in that sort of fashion. The whole stage takes place in a metal-melting foundry...
R-Type II has Stage 5, which includes enemies that spawn both breakable and unbreakable blocks.
They Wasted a Perfectly Good Plot - This series has a truly awesome plot with a man-made Lovecraftian Cosmic Horror as its antagonist, ripe to fill the player's head with Nightmare Fuel ... that they never fully used. R-Type Command does expound on this greatly, though, through the fleet commander's logs.
Made worse that the amount of plot there is...is really good, and most people, even those who play shmups, will probably never learn of it.