Well, we're right in my wheelhouse here.
I haven't had a C5, but I've had a C4 and a C6. A lot of people love them, but I don't care for the way Corvettes feel on track. I like a nice, tight, buttoned-town kind of feeling and both of my Z51 Corvettes felt really loose. You need to be comfortable with autocross levels of slip on a track at much higher speeds. It's stressful to me. Fast, though, but add wheel bearings to your list of consumables, and brakes and tires are not super cheap for this car.
I've had all the M3s except for the new turbo ones. An E36 is a fun car to drive relatively quickly, but once you're at a level where you're really hammering on it, it starts to fight you. As your skill level increases, expect front pads (and we're talking Pagid blacks, PFC-01s, ST-43s) to last less than 2 days and you'll start shredding front tires over a full track weekend because you're alignment-limited with stock parts and the staggered wheels aren't helping. I never had subframe tearing on any of my cars and I tracked them *hard* but it's been known to happen. Oddly enough, my S52 car drank oil on the track but I never had any such issues with my S54 across all three sets of replacement rod bearings. ¯\_(ツ)_/¯ I don't find M3s to be that much fun. I know there are some really big BMW fans, but every time I get an M3, it just sort of does its job. They're impressive and they look cool and they have presence, but I never got out of any of my M3s grinning and thinking, "That was a great drive!" They just aren't fun to me. Every M3 I have bought, I've replaced in a year with another Porsche.
So, now, on to the Porsche... sigh. I love the 996. I really do. An early 3.4L car is probably my favorite-driving 911 out of all of them. It feels so good to manage weight transfer, the steering is fantastic, it so communicative, I love the way they sound, I love the way they look, I love the headlights, and I love the interior. I still can't understand why everybody dumps on the headlights and interior! Anyway... :) IMS is not the issue everyone on the internet says it is. I've had lots of M96/M97 cars and I've never had an IMS issue. I have grenaded a 997 3.6L engine on the track, even with the LN Engineering 2.5q deep oil pan and pickup and Porsche Motorsports air/oil separator. 18k for a used (not rebuilt.. used) engine. The 3.6L IMS bearings are a thinner, single-row bearing and are allegedly more prone to failure, but, again, I personally have never had an IMS issue and I've had some real dogs and I've had some exceptionally nice ones over the years and none have blown up due to IMS issues. They're fast, they're comfortable, they're set up to not be super hard on consumables, even the non-motorsports 911s have some of the best brakes that go on passenger cars, they're actually pretty reliable, but some of the parts are a bit on the expensive side. In my opinion, it's worth it. Early Caymen are the same way. Same basic motor. Maybe an accusump would help? I don't know. When you are really driving the piss out of them, they'll ping and ding and smoke the next time you start it up and that's all normal and expected. Just watch your oil pressure and your revs. Lots of people drive 996s and 997s and Boxsters and never have any problems, but the higher up you go, the more preventative mods there are to help preserve the engine. If you're prepared for that kind of thing, the 996 is so rewarding to drive. Of the three choices, it FEELS the best to really wring out. Tremendous fun. There are more 996s in my future, I know it. :)
I had a Fiesta ST and on the street it was a riot. Super fun to drive, although the Recaro seats ended up absolutely killing my back. Do an extended test drive with both seats before you decide you want one... Anyway, that was probably fixable, or maybe I could have downgraded to the non-Recaro factory seats, I don't know, but after my first track day, I could not get out of that car fast enough. It was AWFUL. The brakes are already too small, and the electronic torque vectoring that you can't turn off MUNCHES on those tiny brakes. They get HOT and they wear out real real fast. And the rear bias is not fantastic - coming into, say, turn 5 at Road America, I have never ever ever had a car that waggled its butt from side to side under heavy braking like that Fiesta did. It was off-putting to say the least. I have heard of overheating issues putting the car into limp mode on the track but I didn't see that. I had terrifying brake behavior and that was enough.
A wildcard choice for you, maybe: I picked up a Mk6 Golf R a few months ago and that thing is FUN. I only just picked it up in February and tracks still aren't open yet so I have no input on it for a track car yet, but, yeah, fun fun fun. I love it.