That´s a tough one. My List would be this:
1. TYRANT
The most classic of RE bosses is still the ultimate badass. silent, brutal and big ass claws. The most awesome version of Tyrant was never in the games however but in the movie Resident Evil: Damnation. The ultimate killing machines.
2. G
The monster that is created by injecting the G Virus into a human host. It has various forms as it mutates over time. This is by far one of the most disturbing and awesome monsters in the series. The design is pretty insane with the human head in the chest, the iconic eye at the arm, the various arms, claws and myriads of teeth on this thing. Also the fact that this thing implants embryos into human hosts that break out of their chests (Hello, Alien!) is as perverted as it is brilliant.
3. VERDUGO
This seems to be a Las Plagas-created creature which has one of the most awesome designs ever. At first we see it , cloaked in a pretty cool robe and hood with glowing eyes and when it´s true form is revealed, people were ****ting themselves. The design is heavily based on the famous movie monsters from the Alien and Predator franchises. The tail is taken from a xenomorph as well as the torso while the face with the mandibles is clearly inspired by a Predator. Also these bastards are REALLY hard to kill as only magnums and Rocket Launcher can pierce their torso. One of my most fav monster designs ever in the series.
4. NEMESIS
Umbrellas ultimate STARS hunting BOW might be the most iconic RE monster there is and became infamous by stalking players and making them crap their pants. The first BOW wielding a weapon and its enormous power and sheer indestructibility plus a really badass design made the Nemesis one of the - if not THE - most memorable RE monster of all time. STARS!!
5. LICKER
One of the most iconic and scary RE creatures, the Licker is actually a further mutation of a Crimson Head zombie who was exposed to a very high concentration of the T-Virus. Later Lickers were weaponized for use in bioterrorism in Africa and Eastern Slav republic among other warzones. They were later even implanted with Plaga parasites so that one being infused with a Control Plaga could actually control Lickers and use them as soldiers. The Lickers are just some of the coolest RE creatures ever with their skinned flesh, large claws, Alien-like jaws, deadly tongue and exposed brains. And who wasn´t scared ****less in RE 2 when the Licker jumped through the window in the interrogation room? Still gets me every time. Hell they are so awesome that even the live action films got them right.
6. REGENERATOR/IRON MAIDEN
Okay these things are by far the scariest creatures in the RE series. The way they move silently but almost unstoppable towards their prey, that evil grin-like face of theirs and the fact that they can regenerate nearly every injury instantly (unless you kill all the Plaga in their body) makes them so damn terrifying. The Iron Maiden version is even worse as he moves faster and can kill someone very quickly by extending deadly spikes from their bodies. If you don´t exactly have the Handcannon or Rocket Launcher at hand, dealing with these things can be extremely nervewrecking. Everyone saying RE 4 is not scary, obviously has never met these guys.
7. HUNTER/SWEEPER
Another one of the most classic RE creatures which are really often the alternative to the Licker. With their deadly jumps, claws and quick movements they can be really devastating (that damn head swipe!) and unfortunately they like to hunt in packs. Wesker uses them as his favorite attack dogs and after Umbrellas fall they were used (much like Lickers) primarily in bioterrorism as deadly footsoldiers (most famously during the Terragrigia panic in 2004). The Sweeper is a particularly nasty variation who is not only tougher but also venomous. To quote Parker Luciani "What a pain in the ass piece of ****" But they look so damn cool do they?
8. EXECUTIONER MAJINI/CHAINSAW MAJINI
While I am generally not much a fan of the gun wielding Ganado and Majini (as RE tends to become much more of a shooter once you have actual firefights with enemies), these things are pretty awesome as they are basically straight out of a slasher movie. the Chainsaw wielding plaga infected wear a bag on their head which looks a lot like Jason Voorhees in Friday the 13th Part 2 and could be demons straight out of an Evil Dead movie. When they rev up that chainsaw and coming for you , you can´t tell me that you´re not starting to **** your pants a bit (especially if your shotgun is empty and you only have some handgun ammo left). The Executioner Majini is an even bigger badass and looks a LOT like cannibalistic killer Leatherface from the Texas Chainsaw Massacre films, only instead of a chainsaw (which apparently the Chainsaw Majini are not willing to share) he wields a big ass axe which can kill anyone pretty damn fast. Those red variations of them are even more nasty. If you don´t have a pretty damn good weapon or infinite ammo and you see that big motherfcker coming right at you, you know what it feels like for those poor bastards who have to go up against Jason or Leatherface in the horrorfilms. Play RE 5 on professional, use no infinite ammo and only a handgun when these big bads are coming for you and then tell me again how RE 5 is totally not scary, I dare you.
9. REAPER
Speaking of RE 5, chances are if you die in that game it was most likely a Reaper who sliced you to pieces. These are without question some of nastiest things in the series. Again like so many things in the series they are heavily based on the ALIEN films. Like the Xenomorph from those films, Reapers come out of eggs and like with a Xeno once they get a hold of their prey its pretty much over as they can kill instantly with their spike like arms. They are also pretty creepy and you get pretty nervous when they are crawling quickly towards you. Not only are they fast, creepy and super deadly they also are very hard to kill as they only have certain weak spots which are small and only appear once you knock them back. Though the Rocket Launcher will blast them with one shot, if you only have handgun or a weak shotgun, these things can make you panic pretty damn good. And most likely kill you unless you are a REALLY good shot. I love their design and their similarity to the xenos from Alien and they always get the adrenaline levels up. especially if you have two of them heading at you.
10. UROBOROS
Weskers ultimate super virus is definitely one of the nastiest and biggest horror elements the series has ever had. It reminds me a lot of the Symbiote from Marvel comics (a black alien substance which searches for a host and is pretty vicious and volatile). A virus that turns pretty much almost everyone into a walking mass of worms and black, evil goo which just eats up everything in can find and can grow massively in size? Definitely the most nasty virus in the series. Again its hard to understand why so many people complain about RE 5 not being horror enough anymore when there are things like chainsaw wielding psychos, Alien-like Reapers that can kill you instantly, Lickers and on top of it this horrific monster which is straight up horror movie material. In fact Uroboros often reminds me on several horror films like John Carpenters 1982 classic The Thing, Deep Rising, Species etc. This thing is extremely deadly, looks pretty disturbing and chucks down BSAA agents like their energy drinks. And unless you have a Rocket Launcher (which kills every Uroboros with one shot) you´re in for a nasty fight. My favorite Uroboros is probably mutated Neil in Revelations 2. It looked like Uroboros had bonded with Nemesis. Really badass - and really damn frustrating on Survival mode.