Vanilla beta. Not sure why I got it, but I was a pretty active WC3 ladder player and forum poster. Been in several WoW expansion betas since and SC2 and the HotS alpha/beta.
I’ve always tried to be really active with giving feedback, and I think that helps in getting invites.
Fingers crossed for Dragonflight.
After being invited into WC:III multiplayer beta long long ago I guess I made it to Friends of Blizzard list, so not including the WoW Open beta…
I have been in every WoW expansion beta, and three alphas, except for one, I believe it was Cata.
I’ll let you know when I find out 
MoP.
Testing the pandaren starting zone was both fun and pain
Mine was mop , but then again everyone got mop lol
I had friends in closed Beta for Vanilla so got to see some of that when visiting them. I played Open Beta, but so did everyone.
I know I played Beta for Wrath… and of course MoP. I got into MoP on the first wave which was nice, but still crowded. I THINK I was in most of them after that? Then it turned into every Beta for everything (Hots, HS, Overwatch, etc.) including Bnet App betas and such. I wrote an initial FAQ for the voice app I think back in the day when the new one came out.
I try not to burn myself out on Betas though or I don’t enjoy the main game.
Looking forward to Dragonflight Alpha or Beta, whichever I get into.
I got one in MoP then I didn’t get one in WoD, found and reported so many bugs that made it to live. Then I got legin, BfA, shadowlands, and BC classic. I doubt I’ll get it again.
Wrath of the Lich King.
My first alpha invite was Diablo 3 IIRC.
Unless you are a streamer you will never get one or your chances are 0 to 0.000000000000000000000000000000000000000000000000000001%.
Never been invited to anything. Only test I’ve done recently was 9.1 PTR. Blizzard does change some things - they fixed Soggodon when his initial iteration was boring, but otherwise meh. Feedback mostly falls on deaf ears it feels like, so why spoil the content for myself? I skipped 9.2 PTR and likely happier for it.
Eh. I’ve received beta invites from Wrath through BfA then I finally opted out so I didn’t get one for SL.
I don’t know how to stream.
Wrath was my first, on a beta invite for addon developers by WoWInterface (before Curse won the official addon hosting site war)
I got a MOP one first…
You guys getting beta invite?
Think legion was my first (didnt apply for any before then) and the only other one i got was a streamer giveaway for shadowlands.)
Wrath. I’ve gotten an Alpha or Beta invite for all but like 3 or 4 expansions. Enough to know that people clamor over invites for no reason lol
Edit - no Beta for Vanilla or TBC…or WoD…I think those are the only 3. One of them doesn’t count cause they let everyone into Beta for MoP I think, but you had to sub for a year.
I know I was in the WoD closed beta but I dont remember if that was the first one, pretty sure it was. I remember spawning at a oversized khadgar just outside what would eventually be the shadowmoon/alliance garrison entrance - I checked out the warlock perma infernal (So cool, if so in the way - alas, gone now) and the druid Saberon cat druid form/talent (another one that bit the dust) but can’t remember much else, i know I didnt do a whole lot in terms of questing cause I didnt want to spoil my retail release.
Mists was first and every expansion since then.
none but i prefer it that way.
i manage to get a bit bored as it is - imagine if i drained my fun ahaead of time in some useless beta 
i had overwatch1 beta early and thats exactly what happened.
by the time the game was out i was already like “meh” and then when they broke my torb i just uninstalled.
they never really listen to beta-feedback either apparently. always ppl yelling that they reported an issue and its still on live after. so i really don’t see much uses to it.
I think WoD and every expansion since then. And I never actually installed it.