Depends on family circumstances, all kinds can work.
We wanted to start trying again when DC1 was 18 months but due to career and financial reasons decided not to. We delayed it until they were nearing 2.5 and DC2 was born when DC1 was nearing 3.5. There are pros and cons to this gap, DC1 is now 4.5 and DC2 is 1.
So far, pros: DC1 was independent in many self care ways by the time DC2 came e.g. sleeping, eating, toileting, going up and down stairs at home, could play by herself a bit for short periods of time or could be trusted to colour in and things without drawing all over the walls and that kind of thing, was very helpful passing me things when I was stuck under a sleeping or feeding baby, absolutely adored DC2 from the moment they were born and always been very gentle and affectionate with them, enjoys playing with them, I think a lot of that is due to them being big enough to understand that DC2 is a baby and so will behave differently and isn't expected to meet expectations we have of DC1 yet etc. So there's not been any competition or jealousy so far. DC1's 30hrs childcare started just before I went down to mat pay as I only got SMP. One year of them both being in nursery means a year of not having to do two different pick ups and drop offs so thats bit simpler logistically. DC1 in nursery 2 days a week so I had 2 days 1-on-1 with DC2 which was nice, but then I did actually appreciate DC1's company the other 3 days a week as I found my first mat leave pretty lonely and boring. DC1 helps entertain DC2 a lot and often stops him crying when I can't! Didn't need to buy a double buggy or another group 0 or 1 car seat.
Cons: DC1 still little and still wanted 1-on-1 attention almost all the time and hardly ever played by themselves for the first 6 months or so til they got more used to it. Was very hard to give them attention and play with them etc when I was stuck with a crying, feeding, or sleeping baby in my arms. DC2 napped particularly horrendously as a newborn partly because DC1's frequent tantrums would wake them up. Sooo many tantrums from DC1 still which on top of crying baby was stressful. Found it basically impossible to cook meals and even making lunch and breakfast meant one or both of them was crying whilst I did it, I'd say this was the case for at least the first 9 months or so tbh maybe even longer. DC2 not a good sleeper so shattered and DC1 went through a bad patch of sleeping a few months after DC2 was born so then me and DH were basically up every other hour taking it in turns between kids which was grim. Two sets of nursery fees for a year before DC1 goes to school which is crippling us financially and meant me and DH have both had to make changes to ours jobs/working hours. Limited 1-on-1 time with DC2 and I do wonder how thet impacts his development as in terms of milestones he's meeting most of them later than DC1 did. Had to use a buggy board a lot in the first year as I don't drive, which was very physically taxing tbh!