Spike Growth creates a 20-foot radius (40-foot diameter) circle which is difficult terrain and causes 2d4 damage per 5 feet of distance that a creature travels through it. Making the assumption that:
- 2 enemies in any given round have to try and move through the area for some reason;
- those 2 enemies spend their entire movement and dash as an action to get through the difficult terrain;
- those 2 enemies have a normal speed of 30 feet, so after accounting for difficult terrain they travel 30 feet each through the area for a total of 60 feet's worth of damage;
Then the spell will deal 24d4 (60) points of damage per round - averaging 600 points over the full minute allowed in your conditions.
You're very unlikely to be able to actually get that performance out of the spell, since it does depend heavily on your enemies making very poor tactical choices, or that you are otherwise able to ensure that they need to keep moving through the affected area - if they stand still or can walk around it, they take no damage.
But, if circumstances are perfect and your foes keep dragging themselves through the thorns, no other second level spell comes close to spike growth's damage. Even if you make the assumption that you only get half the damage out of it and do a total 12d4 (30) damage per round, that's still nearly as good as what you can get out of other optimising other 2nd level spells.
Spike growth is available (at character level 4) to druids, Nature domain clerics and warlocks with the Genie (Dao) patron.
Spike growth's position as number 1 requires a lot of very favourable assumptions about the battlefield and enemy behaviour. An alternative more likely to be viable in practice is moonbeam, which creates a 5-foot radius (10-foot diameter) cylinder that deals 2d10 damage to any creature which starts its turn there or enters it for the first time during a turn.
As per the questions' premise, we'll assume that 2 enemies in any given round will either start their turn in the area or pass through it as a part of their movement during their turns, each taking 2d10 (11) damage for a total of 22 damage dealt per round.
However, that only covers the damage that moonbeam deals on the enemy's turns. If an enemy ends their turn standing outside the area but is moved into or through the area during someone else's turn - i.e. yours - they will take an additional 2d10 damage.
As per the question's premise, we will assume that at least one of the enemies always helpfully positions itself just next to the moonbeam in such a way that, on our own turn, we can push or pull them to force them back into the area. Any caster could achieve that using a shove attack, but a druid could do it more efficiently using the thorn whip or gust cantrips to respectively pull or push the enemy into the area.
Under that assumption, we increase the total damage per round to 6d10 (33) and so deal an average total of 330 damage over the minute that moonbeam lasts.
Above and beyond this, if we're a druid, we also have access to the thunderwave spell, which can be used to push around multiple enemies at a time. If we devote our remaining 6 spell slots (4 1st, 2 2nd, assuming all four levels are in full-casting classes) to casting thunderwave, then on six of those turns we can conceivably force both enemies back into the moonbeam, and so in six of those ten rounds we will actually deal 8d10 (44) damage, and moonbeam's total average damage over the full minute will be 396 points.
(It is important to note that although we can also use an action on our turn to move the moonbeam around, the official interpretation of the rules according to Sage Advice holds that moving the area of moonbeam onto an enemy does not count as the enemy "entering the area", so this cannot be exploited to deal extra damage - to get the extra points out of it, we must rely on forcing the enemies to move, not moving the moonbeam.)
Moonbeam is available (at character level 4) to druids and Twilight domain clerics, but druids have the easiest time maximising its effectiveness by combination with other spells.