Interesting thread, I have a similar issue. I only started in the hobby around 1 year or so ago, within that time I bought...
40x25x25 (25l)
60x30x30 (55l)
60x35x35 (73.5l)
90x40x40 (144l)
Until recently I was running all but the smallest, I'm now running only the two largest. But as soon as I was finishing settling up the 90cm, I was already making a "dream list" of kit for a 120cm - 180cm tank (which I know I won't have anytime soon as my GF doesn't want any in the living room. I enjoy the planning process etc so much, I'm similar with other things I have an interest in though, like building PC's etc.
Eventually I'll probably only run 1-2 tanks, I'd love to have 1 large tank between 120cm-180cm as my end goal though....or so I think haha!
Edit - My 90cm tank defo gives me the most enjoyment, the extra length etc really does help.