For me one of the main properties of a bidding system is its notrump ladder. Around 52-58% of all opening hands are balanced (it depends a bit on your minimum strength requirement for opening and how many semibalanced hands you wish to include), and it is very difficult to win enough on the remaining 42-48% of unbalanced opening hands to compensate for losing frequently on the balanced hands. Having a sound notrump ladder and knowing it by heart is the cornerstone of any bidding system to me. If you wish to open aggressively I think this is mostly reflected in the NT ladder - do you open all balanced 11's? Some balanced 10's? This has cascading effects on the rest of the system, and requires serious thought.
Jan Eric Larsson wrote a number of opinion pieces on bridge bidding system design, and largely I think he made some very good points. He recommended bidding slowly with strong hands, jumping around aggressively with weak hands, and showing natural suits so that partner can raise in competition. These thoughts are not very controversial and almost any system creator will claim their system does exactly this, but I like how he makes this explicit by e.g. making an effort to avoid constructive or semiconstructive two level openings (well, 2♦ and up) and inventing a strong club system without a nebulous opening (though note that the strong club itself is nebulous, as 'open low with all strong hands' and 'always bid a suit' conflicts to a certain degree). Incidentally he also pointed out that if you are going to open aggressively it is most important to have a way to open aggressively with balanced hands, as those hands usually can't back into the auction later so they benefit most from the preemptive effect.
Personally I think strong club is a step above other systems. If you play a simple version you get more bang for your buck than basically any other system, and I think it is actually very easy to pick up for beginners. Regrettably it is not popular enough for them to be able to find a partner after, so I do not recommend it or teach it to beginners. But if it had more representation I 100% would. Furthermore, even after the entry level you can add tools and gadgets as you please, and keep improving the system at a better tradeoff between effort and complexity than many other systems. Strong club systems have many problems (interference with 1♣, some nebulous flavour of 1♦, (slightly) reduced ability to preempt and/or possibly uncomfortable constructive 2m openings) but also do not have a lot of problems that standard systems do, and on balance I think it's an easier framework for building a bidding system than standard. In particular, I think the Auby-Ebenius club set of openings is really sound and relatively easy to pick up, though I'd recommend natural continuations rather than their relay for beginners.
Of the natural systems I really like the unbalanced diamond, balanced club style (so 5542 but almost all balanced hands outside the NT range open 1♣) paired with either Dutch Doubleton or Transfer Walsh. This requires a lot of effort - modern Dutch Doubleton continuations are complicated and highly artificial, Transfer Walsh is also no easy feat (sure, 1♦ and 1♥ can be handled relatively easily, but all higher responses are messy at best), and a balanced club exposes you to complicated decisions in competition so really I'd like to pair this with a lot of transfers and artificial rules in competition. On balance I think this style of system is also really sound, but I do feel I have to put in a lot more work to get to the same place that strong club systems get to. One very pleasant knock-on effect of balanced club structures is that you can more conveniently move your NT ladder around, e.g. play a Kamikaze NT at certain vulnerabilities without having to do an entire system overhaul (you primarily change the 1NT and 1♣ opening).
Lastly I've been looking at strong club with 4cM for a little over a year now, and the results have been beyond all my expectations. The main issue is that it involves the C-word, 'canapé', which tends to end the conversation on the spot. So instead I recommend Auby-Ebenius, or some simple derivative. Incidentally I played the AEC opening set with natural responses recently with a new partner and the system felt really smooth and effective.
As for some other systems, since I'm writing half an essay anyway I'll just plug some more brief thoughts:
- Standard systems with a 4cM: as mentioned previously I think these are rather closer to "4.5+ card major", and they are very similar to 5cM systems if you open four card suits mostly up the line. I think these are completely fine, though I prefer my Dutch Doubleton or T-Walsh too much to include many balanced hands in 1♦, 1♥ and 1♠.
- Strong pass systems: this is probably controversial, but I think they are likely not as good as they may seem. There most likely is some sort of theoretical system out there with pass containing a number of strong hands, but the classical route of pass = 13+, other bids showing 0-7 or 8-12 or something similar comes with a laundry list of weaknesses. I think these systems have limited theoretical merit.
- I am really interested in the Fantunes system. Forcing natural 1-level openings check all the boxes I'm looking for, though I dislike their constructive 2-openings to take pressure off of some constructive auctions. I already play Gazzilli versions over 1♦, 1♥ and 1♠ and Dutch Doubleton over 1♣, all of which have built-in strength splits on the second round. Perhaps these can be modified or stretched to accommodate unlimited forcing openings?
- These days it is becoming increasingly fashionable to swap out preemptive openings at the 2-level for some constructive or semiconstructive ones. My personal results doing the opposite - making the weak two openings extremely aggressive - have been incredibly positive and I would recommend not giving these up if it can be avoided.
- Polish/Swedish club systems (strong club but 1♣ can also be a weak notrump): I think these are markedly weaker than both strong club systems and natural systems. If the opponents interfere with the 1♣ opening responder is not nearly as free to act as in a strong club system (force to game with 8+, make some noise with 5-7 even at the 2- or 3-level depending on structure) as we have to cater to opener having a weak notrump, and conversely opener very often has to act again with a strong hand even at uncomfortable levels.
Having said all of this though I stand by the John Montgomery quote from earlier. An internally coherent system with multiple rounds of the bidding spelled out and practiced will outperform some rough framework with no idea of the followups, and in a similar fashion experts with a poor 'system' will outperform amateurs with a fancy 'system', if that means only the skeleton of the structure. Any of the above can comfortably beat any of the others if a lopsided amount of effort is put into them.
P.S.: John Montgomery's "Revision" is a strong club 5cM system. He claimed much the same I said above - a lot of the problems he tried to solve went away when he tried swapping from standard to strong club, and the problems he got in return felt more manageable.