For me it was the 32 H&R magnum. I didn't like the recoil of the 38 so I bought a new Charter Undercoverette last year. I knew before I bought it that I would need to reload for it so it gave me a good excuse to start reloading. Now I enjoy reloading as much or more than actually shooting.
.308 Winchester, because I wanted to have control over what I hunted with. Back then I had an inexpensive NEF Survivor .308, with the bull barrel. I got some groups under 3/4" at 100 yards with that rifle and some handloading.
Currently, I only really reload for my Springfield 1884 Trapdoor, as off-the-shelf .45-70 that's safe for that rifle isn't easy to find. I have a .243 Rem 700 that I will run out of factory Federal 100grain SP for soon enough, and I already have the components needed for it.
6.5x284 and a close second was 223 both of which I competed in High Power Rifle matches.
I was intrigued at how accurate someone could make them, A few years later If I shoot it I reload for it.
I currently have a 650 and 2 rock chuckers and wish I had a 1050
Started reloading .38 specials with a Lyman hand loader and progressed to a Lee Progressive loader. Now I load .357, 45 ACP and various other pistol calibers. Being a trap shooter, I now load 12 ga shot shells to help offset the high cost of ammo.
Started reloading 12 gauge on a "Texan" reloading press. Friends and I hunted a lot and none of us had any money. We churned 12 gauge rounds out of that Texan by the case! We all used the Texan and we had all the goodies to make field loads and heavy duck/Geese loads.
Not long after that, I started loading .357 magnum for a Ruger Blackhawk and .45 acp for my first 1911. That was 1968.
I started loading .222 rem about 50 years ago, primers were less than a buck for a hundred, 50gr sierras about 2.5 cents and powder was about 2.50 a pound.
When 180 grain JSP for my old Remington Model 760 in .300 Savage went to nearly 2 bucks each, I started reloading again.
(I'm way too much of a cheapskate to pay 38.75 for a box of 20.)
Now, I'm about to have Shilen make me a .400 Brown-Whelen Improved (which is a rimless "twin" of the "well-regarded" .404 Jeffery) barrel for another Model 760, I'll have to load that ammo, as the ONLY current source for loaded ammo is FOUR BUCKS EACH.
Note: I originally started reloading for .220 Weatherby Rocket, .244 Remington, .257 Roberts, .30-40 Krag, .32-40, .38-40 & .41LC as I couldn't find any of those in the little town where I went to college in the mid-1960s.
Moved up to AK for a while in 2000. Federal premium was $36 for 20 rounds at the time on Kodiak island, I was able to hand load 100 rounds for $32 so easy decision there....Soon after that I started shooting a 308 , and various 17cal center fires so hand loading was a must.
First for me was reloading 12 gauge shotgun. Back then, you could save real money doing that, and I hunted and shot skeet a lot. The first centerfire cartridge I got into was for my .300 Savage hunting rifle. What little you could find in stores was very expensive, and most of that was 180 grain. 150 grain was more than plenty for Texas whitetails.
I agree 100% that 150 grain is plenty for Texas WT & exotics BUT my old 760 shoots 180 grain better & it MIGHT be sometimes better for LARGE feral hogs, too.
Addenda: My brother-of-the-heart is casting me VERY NICE 175 grain gas checked FN bullets & considering powder & primer, my 180 grain hunting rounds are costing me about 35 cents each. = That makes for fairly cheap shooting vs 1.92 each for factory loads.
I started on 12 ga in high school. A friend had a MEC 700 and we loaded for all our bird hunting (dove, quail, various waterfowl before the advent of steel shot). I got in to metallic loading in college. .38 Spl/.357 Mag was my first. soon to be followed by .45 ACP.
After college and getting a real paycheck, I got a Dillon RL 450 and Lyman casting set up (1984). I still use the heck out of the 450 and have burned up 3 furnaces. I currently have a MEC 600 Jr, Lyman lubrisizer w/Midway heater plate, Dillon RL 450, RCBS Jr. single stage press, and a Magma Case master (look it up, it is cool). I use a temp mount for a Lee 20 loader as I have run out of room on the bench. When I move in a year or so (after retirement), I will build a bigger bench, in a much bigger shop and have everything accessible.