My experiences so far with a variety of species. My levels at the moment are Eft Lv50, Peiste Lv65, Raptor Lv62, Beetle Lv60.
Lizard
What I started with. Nothing really worth noting here for the first 15 levels, just whack away at stuff. At Lv15 I suggest immediately changing to the "Ashen Lizard", this gives you access to black magic which can speed kills along nicely.
Raptor
Unlocked for 3000 Infamy. You are stuck with being a pure melee for pretty much your entire leveling journey as a Raptor. However, thankfully you do your one job well. Raptors hit hard and fast with Berserk and Double Attack only helping this along further. I suggest purchasing the Dragoon's Instinct as soon as possible for the whopping +40 accuracy it gives as it will speed up kills considerably. Warrior's and Dark Knight's Instinct should be next on your shopping list. When you get to Lv38-40 I suggest heading over to Crawler's Nest. Despite the species advantage you have over most things in the Nest you will still suffer from Crawler's slow which will slow down your kills. Emerald Raptor unlocks at Lv30 but doesn't seem to do anything else apart from give you a new coat of paint. You can pretty much stay in the Nest all the way to Lv60, moving along the food chain as you go. At Lv60 you unlock the Ruby Raptor model which gives you access to that move you oh-so-love from Morimar's fracture: Whirling Inferno. From this point on you might as well stick with this sub-species.
Eft
Plays like a cross between a Lizard and a Raptor and is unlocked at Lv30 Raptor. For the first 30 levels of your life as an Eft you're pretty much stuck being a melee. When you get Lv30 I suggest changing to the Tarichuk sub-species as the magic damage will help speed things along nicely. As with Raptors and practically any other Lizard Crawler's Nest is your playground when you creep up upon Lv40. However, it seems that Efts are EXTREMELY resistant to both Slow and Poison so you can own Crawlers at will. Faculty selection is similar to that of a Raptor but if you want to throw in Geomancer's or Black Mage's Instinct in to bolster your magic then by all means. I took Eft to Lv50 so I could unlock...
Peiste
Words cannot quite describe how much fun you will have leveling this thing. Unlocked by getting both Raptor and Eft to Lv50 and by forking out 8000 Infamy. Peistes are the scourge of many monsters and are death incarnate for vermin in particular.
Despite being limited to a melee-only role and lacking any sort of magical prowess Peistes gain two very good TP moves at a very early stage: Torpefying Charge (1) and Delta Thrust (10). Torpefying Charge packs a nasty paralysis and is a fantastic "Oh ***" button for when you get one-too-many links. Delta Thrust is as frustrating as you remember it but thankfully you're on the giving end of it. Delta inflicts a very strong plague effect on your target which saps its' TP. You really notice the effect of it and, when combined with Torpefying Charge, can utterly cripple anything.
If it seemed impossible Peistes get even more dangerous when they reach Lv40 and unlock what is arguably one of the most if not
THE most broken attack currently in Monstrosity: Calcifying Mist. Calcifying Mist is practically unavoidable by any target, even if said target is a good 20-30 levels higher than yourself. It starts a countdown to petrification that takes 20 seconds to take effect. As the timer ticks down the target begins to slow to a crawl before the petrification eventually takes full effect. The petrification effect can last for upwards of 5 minutes, 5 minutes where you can just wail away on an immobilized target that has no means to defend itself.
Once you unlock Calcifying Mist you can blight the Crawler's Nest like a dose of Colorado Beetle. Much like Efts, Peistes are practically immune to all forms of Slow and Poison so you can terrorize Crawlers to your hearts content.
Beetle
Unlocked for 3000 Infamy. Beetles are Paladins and as a result have a slower kill-speed in comparison to other species but are undeniably sturdier. "Onyx Beetle" can be unlocked by trading x25 Beetle Spirits to an NPC in the foyer, which I highly recommend you do as soon as possible. Onyx Beetles retain the sturdiness of Paladins but add some Black Magic spice into the pot. Given how much Paladin type monsters suffer from a lack of damage output, and you will notice it if you have leveled other species prior to this one, you will want to augment your output by any means necessary.
I'll be frank here: getting Beetle to Lv50 is incredibly painful but when you get it there... Oh man... Lv50 unlocks the "Gamboge Beetle". This sub-species, which looks identical to Lancing Lamorak found in Voidwatch, remains a Paladin but gains the spells and traits of a Red Mage. That means you suddenly gain the ability to cast Haste, Enthunder II, and Cure IV on yourself as well as Dia II everything you fight. If this wasn't enough of a boost changing to this sub-species unlocks an incredible attack called Rhinowrecker. Rhinowrecker is a conal physical attack that hits like Mike Tyson and smashes the target's defense into oblivion. Combine this move with Dia II and Haste and you will begin tearing through things with an ungodly speed.