I only play Noxious, but I'll try to summarize:
Reptile is a well-rounded character with an answer to most situations. His most glaring fault is a reliance on meter to excel (his primary low mixup takes meter to convert, his zoning is mediocre without meter, etc.). His variations aren't as extreme as many other characters, so much of your decision-making comes down to how you intend to use his precious meter.
Deceptive is the most meter-dependent because it has the most it can do with it. Multiple opportunities for frame advantage on block, combo extensions, safe reversals, string-gap punishes and more thanks to the high speed and armour from EX dd3. The trick is that, combined with anti-air/reversals/breakers, Deceptive runs the highest risk of running on empty in a critical situation--especially since you really want to use EX dd3, not meterless. Even so, the flexibility provided makes this a very strong choice.
Noxious is the least meter-dependent in turn, mostly because his EX dd3 isn't great. The passive fumes are "okay" on their own--they slightly increase BnB and chip damage, and meterless dd3 improves it. Unlike other variations, the fumes stay on block, so it has more staying power. Combined with the slight damage they take when hitting you, Noxious is useful for close situations and shutting down comebacks. You'll be sitting on more meter for anti-airs and stuff, too. The downside is that this Reptile lacks anything to make him stand out. No special frame advantage or extensions (with one notable exception). Probably his weakest variation.
Nimble is meter-dependent as well, but is different from Deceptive in that EX dd3 only allows combos from certain situations. It's less flexible, but it trades off in an obscene punish game, deadly corner carry and combos, and options for armour-breaking oki. The biggest downside is that meterless dd3 isn't very good, so if you want to get any of the scary stuff you'll be spending meter (and using f412 a lot). As a combo-heavy variation, Nimble asks for lots of situational awareness to maximize your punishes, corner carry, and potential wakeups--and since you can't block in Basilisk, you risk more with a mistake.
Combo Starters:
2124 (Hit-confirmable, -8 on block unless you stop at 21. 21 is High/Mid, so be ready if they duck)
f412 (Fastest punisher, Mid, leads to good damage. Very important for Nimble)
b1d4 (High/Low that combos into df3. A bit easy to react to, but has its uses. Especially in Deceptive)
b3 (Low that combos into EX db2 for extension. b34 is the meterless option for much less damage. -8 or worse on block)
f21, 1+3 (Hit-confirmable advancing Mid/Mid with MB extension. The 1+3 is death on block; watch yourself)
b2 (+1 on block overhead, so damn good)
34 (Slowest punisher, good damage. Excellent meterless damage in Noxious)
EX db2 (Not recommended raw, but if you reversal with it or whatever it launches)
EX db4 (See above)
EX bf4 (In the corner)
Forceballs (duh)
Mixup:
Standard stuff. Get advantage, mix up b2/b3.
b2 is the overhead of your dreams. Fast, plus-on-block, pushes back to a good distance, and starts combos. Without meter, this is what you want, but your opponent will be looking for this. Mind its short range--it's not Cutthroat Kano b1 bad, but it's close.
b3 is a solid low in general (long range, safe, tripguards, etc.), but it's also your chief low starter. b1d4 isn't a reliable way to catch someone blocking high, and as such you'll sometimes have to risk b3xxEX db2 to open people up. It's slower than b2, so you'll need more advantage or a hesitant opponent to land this clean.
21 is +2 on block, but it's hit-confirmable with 2124. Since they can armour out of your b2/b3 mixup after 21, 2124 can be used as a gutsy callout. Not something to rely on, but it's there.
f21 alone is about -4 on block, and can hit-confirm the 1+3. I wouldn't advise the same trick as 2124 with this, but you can try using EX db2 to catch someone trying to use the plus frames. In general EX db2 can be used at small disadvantages for abare.
d3 is your fastest normal, so you can use it to escape pressure and start your own mixup or as a fast poke in footsies (though its range is very very short).
d4 is longer and slower than d3, but gives huge advantage on hit and is +0 on block. If it hits, a run-in mixup is free.
As always, mix in throws as required. Reptile's aren't anything special, but you can't not use throws.
Other pressure/mixup situations call for something other than Noxious, which I can't help you with.
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